Erase all backups before using HDD for two TM backups?

AFAIK you can just erase some of the older backups to make room for the backups for the MBA.

If you have the money I would just buy a second external hard drive instead so theres more room for the MBA to back up to but if you want to have both on the same drive just delete some of the older TM folders.

 

You really do need a larger drive if you can afford one. Roughly speaking you need TB for TB, so if you are backing up 2 Macs with a total of 1.5 TB you need at least that capacity (preferably more) for your TM backups.

Remember that TM will delete the oldest backups when your backup disk becomes full. The oldest backups are replaced with new ones so you never actually run out of space, but you might find you do lose data you might one day need to restore.

I have a 3TB Time Capsule which backs up my MBA and MBP. A Pegasus R4 takes care of my iMac. HDD's are pretty cheap, and USB caddies can also be used fro Time Machine...Fast? no, but it's an inexpensive way of achieving reliable backups.