I really doubt the weight limit has anything to do with the seat post. Rack limit is almost certainly imposed by the manufacturer and relates to the rack itself and/or its clamp. Fully circumferential clamp requiring
seat post removal to install, would be considerably stronger than one mounted with a separate plate, but
both would be more than strong enough for a 25# limit rack which applies to the rack itself. Likely you could exceed this by 25-50% before any real concern about breakage, which would not be the seat post.
ATB posts are rated at least 2x body weight, dynamically and the essentially static load of the rack will
not stress them at all. A bit puzzled how you would mount two racks- 45-60D angle between the racks?
Have to get used to mount/dismount by stepping through and much weight discrepancy on one side might
futz with ride dynamics.