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!