I'd wait until the banana wine is completely finished and clear before blending. The peach will be fine aging during this time.
It's really not ok to use a 5 gallon carboy with a 3 gallon batch. In primary, it'd be fine, but you really want to minimize any headspace after that. I'd suggest either a 3 gallon carboy, or three one-gallon jugs for a three gallon batch.
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