Time Capsule and multiple backup drives
Golden Rule of Backups: Keep it simple!

Swapping disks and storing one of them off-site is a good backup strategy. But it also means that there are some manual steps every time you swap disks and sooner or later you will get it wrong. To avoid unnecessary data loss you don't


use one external backup drive for several computers
mix backup and other use on the same disk


Given the low costs of adding two additional hard disks to your pool I don't think it's worth the risk to avoid these costs.