Well-made app, simple, and straightforward. A few suggestions:
1. Ability to play polyrhythm in background while using a different app
2. More than two rhythms (e.g. 3:5:7 or 3:5:7:11)
3. increment/decrement tempo with plus/minus buttons or (even better) auto accelerate/decelerate
4. When only two rhythms, option to assign one rhythm to left audio channel and the other rhythm to right audio channel. This would help in training the left hand to sync with the left ear and right hand to sync with the right ear.
5. Option to make sounds more like clicks rather than drums because, when the tempo is very fast, clicks would be easier to hear as distinct events.