you can divide the drive into two partitions - one for your imac backup and one for your macbook backup. then they would be easy to distinguish the two. but no distinguishing needs to be made as time machine automatically chooses which to backup where and which to access when engaged.
although you could do that it could waste space depending on how much changed data you have on each machine.
The best solution is to keep the drive attached to your iMac and in system preferences -> sharing -> files sharing add privileges to your drive for your user. on your MBP go to the time machine preferences -> choose a drive and you will see the USB drive attached to your iMac. keep in mind that to back your MBP your iMac and your USB drive will both need to me on.
"The best solution is to keep the drive attached to your iMac and in system preferences -> sharing -> files sharing add privileges to your drive for your user. on your MBP go to the time machine preferences -> choose a drive and you will see the USB drive attached to your iMac. keep in mind that to back your MBP your iMac and your USB drive will both need to me on."
__________________
Hi andyboutte, thank you very much for this advice. I can surely use this in my own setup. Have a nice day!