Those are the levels I dindn't manage to do with only 2 demons:
10, 12, 13, 17, 23
And of those I think 10, 17 and 23 are impossible to be done with only 2 demons (for 12 and 13 I won't make any affirmation but they most likely aren't solvable with 2 demons too).
If you want I have screenshot of each level I solved using only 2 demons to prove my claims.