I have mac pro and an iMac desk top. can I use same external to back up both at different times

¿are you running MacOS 10.13 High Sierra or better one each Mac?

if yes, you don't even need to move the drive around, you can just share it over your Network:

Use a shared folder with Time Machine on Mac - Apple Support

No partitioning or complicated foolishness required.

As the article explains, you just share a folder, and in the Sharing > File sharing pane you add attributes that say it is a shared backup folder, and then tell Time Machine on whatever Mac to use it.

Time machine backs up every Mac-formatted drive connected to your Mac at backup time, unless it is on the [ONE] exclude list. If you want two waves of backups with different contents, you will need to use a different backup method for the other one. Readers here suggest CarbonCopyCloner, SuperDuper, and occasionally ChronoSync.