How can I serve banana flambée for the next day?
Short answer:  you cannot reasonably make bananas flambee ahead.

While it would be conceivably possible to prepare your caramel and cook the bananas through the day before, you would then need to refrigerate them over night.

Reheating them would be tricky, and might even take more time than preparing them in the first place--and you would not want to serve them without reheating, as the dish would be gloppy and dissonant between the hot parts from the flambee, and the still cold parts.

This is a dish best served a la minute, that is, prepared at the moment of service.   There is little prep work that can be done the day before that will save you significant time.