@Cedric I would avoid keeping the Sparsebundle backups on the same hard disk as the OS,, Time Machine was updated to include Delta Backups for some file types (thouse that support versions I think, rather than all files), and as such a single change to the sparsebundle could require rebacking up the whole file each time.
My suggestion would be to get 2 external drives (of approx twice the size of your combined individual iMac/Macbook drives if possible. Set Time Machine Server to backup to one of these drives as a single partition which will create Sparebundle backups per machine backed up. Keep this drive plugged in at all times and keep Time Machine running regularly. This is your goto backup.restore drive. Then, clone this drive to another identical drive periodically (depending on how sensitive you are - bear in mind that this backup is your go to drive only if both the original machines and primary backup drive are lost at the same time, i.e. fire or theft rather than hardware failure).
There are many ways to acheive this, Carbon Copy Cloner is a nice one as it will perform a full clone the first operation, and then only do the changes in subsequent operations making it a little quicker than cloning the whole thing at one (Which I belive is what SuperDuper will do, I'm not aware unless it has been updated that it will do incrementals). I actually use Chronosync for this task, but only because I already own it, and it does the same thing (although I believe CCC is faster).
Hide this disk in a seperate location to the Time Machine Server and primary backup disk. There is no point stacking them together and losing all 3 if you get robbed). I am providing this an an answer even though the previous anser has been accepted purely because it is way to big to be a comment, please do not vote on it.