Two macs backing up to one Time Capsule, first Mac hogging all the space
There is nothing stopping you from deleting some of the older backups, so long as you do it safely.  After all, you're not likely to need 2yrs worth of backups. 

Follow these steps to safely delete backups:


Enter Time Machine (on the Mac whose backup you want to delete)
Go to the point in time you want to delete
Click on the cog icon in the Finder and choose 'Delete Backup' (depending on your version of OS X you may need to right-click in the finder window to choose 'Delete Backup')


By deleting backups this way you ensure that Time Machine keeps an accurate record of your backups and keeps your data safe.