If you don't mind your "couple of other fish" being eaten eventually and a slight element of risk it is possible to mix one of the more reclusive piscivorous eels with a "pebble tooth eel". Having the more aggressive being smaller helps. Twice I have seen large-ish (30" or so) zebra eels with a yellow-head moray. But the yellow head is fairly tame as compared to other larger species. Two aggressisives though, means trouble, ime.
How about a pair of zebras?
hth