This is interesting and timely as I just asked a client to do this...open two contracts. It's because I need them to escrow money for me to pay for something and they want me to work hourly. I don't know of another way to handle this situation. Usually I wrap the entire amount I'm working + other fees into a milestone, but this is an ongoing project that will work better hourly. I wish there was a contract type that merged these types of payments.
Maybe I'll have to rethink.