I have a job I'm looking at which has an existing 400 amp service which splits at the meter to (2) 200 amp fused disconnects and then feeds (2) 200 amp panels independently that are ion the second floor in the middle of the building. They want a 80kw generator to do the whole service with a 400 amp x-fer switch. I'll probably need to go to a single 400 amp service disconnect ahead of the x-fer switch. Space is a real issue and cost, so what's new? My questions are,

The existing service conductors and feeders pass each other in a box mounted below the 400 amp meter but are not in the same "cable or raceway" as the code reads.They are in a box so I should be able to leave them, correct?

Splitting the 400 amps after the x-fer switch? Can I use the 10 foot tap rule , put a box at the panels, splice the existing (2) sets of 3/0's, call them parallel's, and tap to each panel with a set of 3/0 into the main breakers of the panels?

Please help. I think I've done my homework. Please let me know.