I don't know for sure, but if I'm not quite mistaken, a Priori Incantatem event between two unicorns could possibly cause extremely painful auras to follow literally seconds after the connection is made. The reasoning behind that is simple.



According to the book "Harry Potter and the Goblet of Fire", as well as seen in the films, the magic chain caused the wands to vibrate at a painful frequency to the point where it caused straining of magical energy and struggling to hold the wands still even. In this sense, if a connection like that happens, it could result in pain equivalent to that of an aura migraine as well as potentially developing symptoms of a concussion due to the tremor-like vibrations spreading from the horn and into the skull.



In essence, if a Priori Incantatem event did occur between Twilight and Trixie, they wouldn't last more than 10 seconds at best before they would need to be rushed to the nearest hospital.