SegWit2x is a combination of both SegWit and a 2MB hardfork (to activate three months after SegWit).

SegWit2x uses a different 'bit' for signaling (bit 4 instead of bit 1) than SegWit.

SegWit (BIP141) is not activated at the moment because it requires 95% of the mining hashrate, and not enough miners support SegWit at the moment, because some miners want a hardfork to increase the blocksize. To work around this the proposal of BIP148 allows users to force the miners to mine SegWit blocks by orphaning other blocks after August 1, 2017. BIP149 is a similar proposal, but will activate 11 months later. The vast majority of the core developers prefer BIP141 but most find BIP148 and BIP149 acceptable alternatives.

SegWit2x differs in implementation because of the activation of the 2MB hardfork and the lower activation threshold of 80% of the mining hashrate. SegWit (BIP141/BIP148/BIP149) itself has been developed by dozens of developers for over a year, has been tested extensively and has been ready for activation for some time now. It also has been successfully activated in Litecoin already. SegWit2x however is being developed at the time of writing by just a few non-core developers, although SegWit2x aims to be 'minimalistic', some people still consider this a risk because there is only a few months for development and only 2 weeks for testing.

At the time of writing the percentage of blocks signaling support for SegWit2x is at 78% of the mining hashrate: https://blockchain.info/charts/nya-support