Your conscience is causing you to second guess yourself....you're commited to two agents at this point and are reluctant to nitify the first agent that you have moved on....there's nothing wrong with this. Simply email the agent making her aware of your need for an earlier response.
If your choice is to work with both agents it would be best to be up front making them aware that you are working with both. It's the right thing to do and may cause one or the other or both to decline the opportunity.
You know the right course of action.....it's a matter of doing it.