When I use pallegina and my main char paladin and both cast sworn enemy on the same target, the first use will be surpressed by the second one according to the enemies information tab.

I didn't check whether it is just a problem in displaying or if it actually denies the boni to the first character using the talent, but to my understanding, it should stack either way.

Yep same with a rangers marked prey. Which ever mark goes on second suppressed the first (sworn goes on first marked prey will suppress it). Likely won't be fixed at this point.