Two players, one phone, no second device and no room code. CYDI's 2 Players mode is pass and play: you hand the phone across between turns, and it tells you whose turn it is so nobody sees anything they should not.
Both players get the same shape in a round. On your turn it appears for three seconds, then vanishes and you have twenty seconds to redraw it from memory. Neither the other player's drawing nor their score is shown until you have both finished - so the second player has nothing to copy and no target score to aim at.
Once you are both done the round opens up: the shape you were given, both drawings laid over it so you can see who got closer, and the accuracy and speed behind each score. Whoever starts alternates every round, scores add up, and the highest total at the end takes it. It works the same in a browser or in the Android app, and needs no connection once the page has loaded.
Enjoy CYDI on Android - get CYDI on Google Play