Doubling — Delaying a signal and sending it to the second channel will effectively decorrelate it if the delay is long enough. Shorter than about 10 ms, and excessive comb-filtering will be heard. Longer than 100 ms, and the delay is too audible. Around 20 ms works well.

Stereo Delay — A variation of the previous concept is to create two different delays and add each to the original signal to create a stereo output. Longer recirculating delays can be used.

Chorus — Changing the pitch of a signal will also effectively decorrelate it, provided that the amount of pitch change is sufficient. Chorus has the added bonus that it also slightly delays the signal, and the combination of delay and pitch change makes for an effective decorrelator. Sending the unaltered (dry) signal to one channel and the chorus output to the other will result in decorrelated outputs.

Stereo Chorus — Like the previous scenario, except two chorused signals are generated, each with different pitch modulation and/or delay times. The two chorused signals are added back to the dry signal to create stereo outputs.

Stereo Reverb — Reverb is a combination of many recirculating delays and makes a very effective decorrelator.

Stereo Enhancement — A specialized algorithm designed not to sound like an effect per se but to only give a sense of width to an otherwise mono signal. These algorithms typically use a variation of one of the above techniques.

Don't use amps or speakers of different types.

Don't split the signal chain and use different effects/pedals on each channel.

Do use amps & speakers of the same type on each channel.

Do maintain a single signal chain from your instrument through your mono effects.

Do use stereo effects at the end of your signal chain to create the stereo signal.

I can already tell that I'm going to catch hell about this topic, but just hear me out. There are "right" and "wrong" ways to construct a stereo signal chain. I will try to explain why as simply as I can, but it does get a little technical.Please note that just because you may be doing it "wrong," as I claim, it may still work for you; but this is probably a serendipitous result of trial-and-error and will be less tolerant to changes in your signal chain. However, if you follow the straightforward outline that I provide here, you will more than likely end up with a great-sounding stereo rig without any weird issues.I will try to keep this general, but it's really geared toward the musician who uses pedals and amps. So, here we go...I know what you're thinking. "What's so hard about running stereo? It's just two amps." Well, yes and no. Simply having two amps is not necessarily stereo. If you run the same signal to both, it's just dual mono.What about two different amp types? Ah, well we'll get to that and why that's a bad idea. But first, we need to cover the basics.Let's start with a definition: Wikipedia defines Stereophonic sound, or simply stereo, as “a method of sound reproduction that creates an illusion of multi-directional audible perspective.” More simply put, stereo is sound that appears to emanate from multiple directions.Using only our two ears, we are—to a large extent—able to determine the direction of sounds. So, in theory, using two or more loudspeakers, it should be possible to create the illusion of sounds emanating from any direction.The location of the loudspeakers relative to your ears will have a huge effect on this stereophonic illusion; for example, headphones will be able to create a much better illusion than a pair of speakers spaced closely together. This is because, with headphones, you’ll hear only the right channel in the right ear, and only the left channel in the left ear. You’ll also eliminate any room acoustics that may affect the sound. This results in a highly-controlled listening environment that works very well for stereo imaging.On the other hand, stereo loudspeakers result in some amount of crossfeed, or the blending of left and right channels. This is because the two loudspeakers are in a common acoustic space; so your left ear will hear some amount of the right channel, and your right ear, some of the left channel. Another way to look at this is that the two channels will partially sum within the acoustic space. For this reason, it’s important that any stereo signal be mono-compatible: in other words, that the sum of the two channels does not result in any irregular sound.Depending on phase differences between these two signals, interference can occur constructively or destructively—or more likely—a combination of these two; because phase differences commonly vary with frequency. A simple delay or filter will result in a frequency-dependent phase shift, so any difference in delay times or filtering between these two signals will result in some amount of interference.And this brings us back to why it's a bad idea to use different amp types—or, for that matter, different pedals—on each channel in an attempt to achieve a stereo sound. Different amp types typically have different tone stacks, different pre/post filters, different speakers, etc., which all result in different phase responses. The acoustic sum of these different phase responses may have an unexpected effect on the overall frequency response, due to a combination of constructive and destructive interference. In addition, the different phase responses themselves can result in an unexpected directivity ; i.e., the signal is louder on one side or the other.So, in order to achieve good stereo sound, the signals driving the loudspeakers need to be manipulated in some way to create the illusion that the sound is emanating from multiple directions yet still be mono-compatible and not affect the directivity. How is this possible? The key is decorrelation. In laymen's terms, decorrelation is any process that makes a signal sufficiently dissimilar with the original signal. For our purposes, the signals need to be sufficiently dissimilar that any interference between the two signals is audibly neutral. I just made up the term "audibly neutral" to mean that any interference should be anticipated and unobjectionable, and that the resulting signals and their sums have the same average frequency response.There are effects that are designed specifically to decorrelate a signal from its input, or to create two decorrelated outputs from one input, in such ways that are (more or less) audibly neutral:The result (provided that the effect algorithm was properly designed) is one that sounds balanced and will sum to mono with no ill effects. An added advantage of decorrelators is that they can be placed in series (stacked) without interacting with one another.So, we can finally get back to the question at hand. Let's start with what not to do:What is the "right" way to achieve a stereo rig?What if you have two amps and want to use the effects loops? In this case I recommend that you run the effects send from one amp to the mono input in your pedal chain. Then, run the stereo outputs into the effect returns on both amps. So, you'll be driving both power amps and speakers with one preamp. This ensures that you are not summing two preamps into the effects chain, which can be unpredictable if both preamps are not set exactly the same.If you do insist on using two different amp types for your stereo rig, I suggest using a doubler or chorus to fully decorrelate the dry signal. This reduces the potential for interference between the amps due to having different phase responses.