I already have a external drive connected to my iMac, can I use the same drive to backup my MacBook pro

The amount of data stored by Time machine keeps growing. So using one drive to cover two computers can make backups more complex when the drive nears fullness.

Time machine intends to consolidate older backups and remove redundancies when the drive gets nearly full, but this can result in an unbalanced amount of backups for the multiple systems being backed up. read

Time machine keeps:" below.

Some readers suggest the drive should be partitioned into two Volumes, one for each system, to maintain fairness. You need to do this before a lot of data is saved, because re-partitioning a drive with data already saved often erases that drive (as a side effect).

You can connect as many External drives to your Mac as you can supply with places to plug in, counting extending ports with Hubs and Docks. That's a LOT of drives!

No need to limit yourself to one at a time.