What happens when my time capsule is full?

Time Machine will selectively delete your oldest files.

Using Time Machine the oldest backups are deleted when your disk becomes full.

See Pondini's excellent guide.

This only seems to happen for the Mac you actually want to backup. If you have backed up an additional Mac, it won't delete those backups, so it can say it's full although it could delete old backups from the machine you are not actually working with. Pity it doesn't give you the option.

My example is: MacBook Pro 250 GB, and MacBook 160 GB. There is no reason why there shouldn't be space anough at any one time. Or am I wrong?