It's even easier than that with SnapperMail. While you are on the message composition screen, there is a dotted box around the word "From:" for that field. Tap the word inside the dotted box, and that will allow you to change the account that your message is being sent from on the fly. Provided you have the proper account settings (i.e. Email address in the Identity tab) for each one, returned/replied messages should come back to the proper account.

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Michael