I would be careful with magic weaknesses, as magic is extremely common. 

Splicing a sentient with a non-sentient results in people like Alice, distrusted mutants with some kind of physical advantage coupled with a poor magical resistance. However psychological effects are nil. It is also frowned upon to do so, which means it isn't very common.

Splicing two sentients, is very, very bad. The two (or more) souls often mix improperly seeing as how even people with the same element have very different souls. The consequence is a constant war for dominance that tears the mind apart. This doesn't result in multiple personality disorder so much as violent incoherent insanity, and while the subjects usually mutate into absolute physical powerhouses or conduits of magical power a common result is loss of fine motor control and a sense of self.

Tl;dr They devolve into mindless (though powerful) beasts.