Can I back up two macs to the same external drive using Time Machine ?

Hi,

I believe you can if you partition the drive and use a different partition for each Mac. It would act like two separate drives. Of course you'd have to dismount and disconnect it from one Mac, then connect it to the other Mac each time you'd want to switch backups. Much better to buy a second drive so you have one for each Mac and can just leave them connected and backing up hourly as Time Machine does automatically.

Good advise from previous posting.

Just adding - ALL the EGGs are in one basket. Meaning External Drive do get corrupted and or failing time.

In your setup, should this drive fail - the Backups for both computer will be lost..

The begs the question - just how important is the user data to you ?

Do you have a Rescue Plan In-Place. This comprises having 3 Backups using 2 methods and 1 Off Site incase of Theft, Loss or Natural Disasters. A Time Machine Backup  is very useful and can be used to Revert to Previous Working macOS. 

For addition purposes - two Tested Bootable Clones each to separate external drives. This is insurance incase the upgrades goes sideways. At least one Tested Clone and / or Time Machine Backup should be Off - Site