Hombre may well be correct with the thymolated syrup feed, but I prefer to let them feed themselves, too. No chance of supers (or even extracted brood frames) containing anything other than 'proppa' honey if nothing other than nectar and pollen has entered the hive!
As long as there is no other problem it should make little difference. Nosema comes to mind as one possibility.
I have 4 brood boxes and 8 supers
This is the new kit to make up, OK. If it includes your present two broods, you need at least another one for back-up. A couple of supers would double temporarily as a brood box (almost perfect for me as I am on 14 x 12s).
You will probably need extra broods (or supers for brooding), if they get strong, so make up all 4 broods and be ready for all options. I doubt you will need ten (or more) supers this year if you split both colonies - well not early on, anyway.
I would take the early flow and then split by whatever means you choose - A/S would be most appropriate, I would think. But watch out for early swarming if your colonies are strong and, particularly, if not enough brooding space. Both parts should be strong before any wasp troubles.
Again, as per Hombre, get that kit together, including frames (but without foundation, until required). It is surprising how quickly it can turn into a 'rushed job'.
You could pick your preferred queen and make sure queen cells are from her eggs. you could split just one hive for increase and pile the supers on the 'production' hive.
All sorts of options. Keep them open for as long as possible, is my suggestion. That way you still have a choice!
Regards, RAB