"Dear XX, please tell me if you want to do x or y."

No response, resends email 2 or 3 times

Sends Invitation "Meeting to Agree whether X or Y."

XX replies to original email with an answer

Meeting is cancelled

Nothing like the threat of a meeting to inspire action.

My former boss wanted to have regular meetings because otherwise it is too easy for cross department projects to fall behind. Nobody prioritizes it, so things will not get done. It (should) cause some amount of embarrassment to go to the weekly meeting for N weeks and say you have still not done X task.