Time Capsule and multiple backup drives Right now we have a single iMac in the house and I have two external hard drives.  

One is connected to the iMac and used for TimeMachine backups.  The other is a few miles away (thanks Mom!) to act as off-site backup, and I swap them once every couple of weeks.

Thinking of adding another computer to the mix and I don't necessarily want to purchase two more disks to keep it backed up, so I am considering upgrading to TimeCapsule.

But it has an in-built disk.

So how do people either do 
a) an offsite backup
b) swap disks like I used to
c) anything that will work similarly!
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.