That's a real shame - I have been housing a 7' endli with a 2' senegalus for about a week now. They're getting along fine. I assume it's because I'm keeping them well fed. Interestingly enough, when I first introduced the senegalus into the tank, the endli gave it a few sniffs (it had many opportunities to gobble the senegalus up) but declined to chomp down. I'd like to think that polys wouldn't eat their own if given a choice/are well fed.
Of course other hobbyists here may have differing experiences