Media Backup

I have multiple Optiplex 3020 and 7020 machines and want to create media recovery on one external drive.  Can I create just one for the 3020 and another one for the 7020 or do I have to create a backup for each machine (was thinking of the license keys).  I created one partition for the 3020 and one for the 7020 but need to know if I have to do one for each machine.

Thanks...