On my experience with Flame Trees in my region, is that timing mathers a lot.
I've got some seedlings last mid summer (around Jan/Feb Southern Hemisphere), and the trees grow really fast, of course those on bigger containers grew more, but it was kind of late, as autumn was near, so they started to loose leaves, which is normal, since they are deciduous (or semi) around here, and they dropped 100% of their leaves at mid winter.
I was kinda of stubborn, I thought they could handle our winter that barely reaches 0ºC, and let them winter outside.
(There are many planted on the streets, but yes, on the ground is different)
Of the aprox. 15 trees, 4 didn't survive winter.
Also, you must watch your watering on autumn/winter, they can rot easily if keep it too wet.
Having that said, I don't think you should do it until spring, it may yellow it's leaves and/or drop soon...
When it wake up from dormancy, you can cut it's taproot and repot them on bigger containers. (well draining soil).
Plus, two seedlings/saplings at the same pot it's ok at the beginning, but even if you want to fuse/merge them, I think you should grow a few years independantly, so they could thrive the maximum possible before going dormant again.