SINCLAIR;
The IHC-Atlas is indeed a kit, as opposed to the MTH which I believe comes fully assembled. The long walls of the IHC-Atlas are comprised of 4 sections, so they can be shortened. With two kits you could lengthen it as needed, as you said. Another option for a single IHC-Atlas kit would be to lengthen one stall with a kit-bashed stall extension. I think this is akin to what UP actually did in real life to accommodate BB's.
Rod