Wording an offer on contract

[Tom C]

I currently have as offer out on REO and have been waiting 4 days for the bank to respond. There is another REO that just hit the MLS that I would like to make an offer on, but I can only get one. How do I word the contract that will allow me to back out of the second bid when and if this first one is accepted by the bank?

Or do I have to wait until the bank makes up it's mind on first one, before I can go and write more contracts?