The issue with testing multi-image with Flux is that it's expensive due to its pricing scheme ($0.015 per input image for Flux 2 Pro, $0.06 per input image for Flux 2 Flex: https://bfl.ai/pricing?category=flux.2) while the cost of adding additional images is neligible in Nano Banana ($0.000387 per image).

In the case of Flux 2 Pro, adding just one image increases the total cost to be greater than a Nano Banana generation.