I (C) want to manage an escrow between A and B.
A deposits lumen into a new account and wants B to do something. If B does something - which I determine if B has done it, e.g. answer a questionaire - B should get the amount in the new account.
For financial reasons I don't want to touch the lumen, which would be easier (I take the lumen and when B has done it, I transfer the lumen to B).
So I think an escrow account would be great. I've read about escrow in Stellar and it seems to involve multi sig transactions. How would this work in this case? How can two people sign an transaction from two different wallets? Should A and B sign this instead of me?
All examples I've seen on escrow assume the private keys are shared by one entity e.g. 
which doesn't look like escrow to me at all.