Assuming two 3+ save models take four wounds...
If they are identical (two or more saves have to be failed for both of them to die): 11/27=40.74% probability for both of them to die (if I didn't miscalculate).
If they are different (both of them have to make two saves; in order for them both to die, both have to fail at least one of these saves): 5/9 probability for each of them to die individually, so 25/81=30.86% probability for both of them to die.
So, yeah, it improves your chances quite a bit. Though it makes the process of rolling saves for this unit incredibly tedious.