Cam spider boxes aren't really combining separate services together, and no, you cannot combine 2 separate 200A drops together to make a 400A
drop. This is for fundamentally the same reasons you can't do a reverse
two-fer with dual 15A male connectors on it to make a 30A
drop. Without getting too far into the mud on why that is, consider that 1) multiple
power generating sources wouldn't like be shorted to each other, 2) if one input was turned off, the other input and everything upstream of it would be backfed and remain hot, and 3) you've compromised the ability for the overcurrent protection to behave as intended, by spreading a
load out across multiple branches of
circuit breakers and disconnects, making it unlikely for your overcurrent protection to keep you and your
venue safe if something unexpected should happen, and 4) there are other reasons.
All those boxes are doing is giving you multiple inputs in parallel with one another, intended to be
fed from a single, common source. So let's say you've got a generator kicking out 800A/
leg. You need some heavy copper to run 800A, which at that
point is basically copper bridge-suspension cable. Not really
practical for someone to drag around unless they're dragging it with a truck. So they give you multiple taps off of the generator that you can run in parallel with common cabling solutions a mere mortal can handle. So you're not really combining 2 services of 400A together. You're loading an 800A
power source across 2 sets of cables for 400A.