These two don’t combo as I’d hoped. Sacrifice doesn’t trigger Omnath’s 5 damage. It was a waste of money for me. If you’re interested in this combo don’t bother.
That must be a bug, right?
By sacrifice you mean at the end of turn or manually replacing?
The former is probably a bug
The latter is probably working as intended.
Probably being the keyword in this game 
Between the two of them you’d get 2 elementals. The second one would have to replace the first to enter the board. It does not trigger the 5 damage unfortunately.
Thats not sacrifice or destroy… i think thats exile