If your back up offer has not been accepted by the lender of the property you should be good to go. It important that any offers terminated prior to the mutual acceptance is done immediately to protect your earnest money.
That said, it would be wise to be writing these simultaneous offers with the same agent as they will be the first to have new information to relay to you so you can make the decision to pull out in time for the reasons stated above.
Refer to your agent regardless.
I believe that as this market continues to gain steam, we will be seeing this more and more.