Most time efficient way

I have a C8 pro with about 30 devices (mix of zwave and zigbee) due to new buildings on property I now have a second hub C8. Given the ranges of some of the links I want about 15 on 1 hub and 15 on the other to be closest to radios. The hubs will be hubmesh together
Does that mean manually un-including then including to a new hub 15 devices one by one or is there a faster way?

That would be the best approach.

Restoring a cloud backup from one to the other, and then deleting half on one and the other half on the other might work… but could also lead to unexpected results…

Agree splitting devices is best and yes remove & re-pair. Keep in mind you should split your repeaters evenly, assuming the 2 environments are similar, as well as device distances/obstacles between hub and devices, to avoid potentially weakening one of your hub's mesh. Or maybe 1 hub setup doesn't even need the repeaters, depends on environment.