I'm interested in buying an external hard drive. I'd like to know whether if it's possible (and simple if so) to have two separate Time Machine Backups from two different MacBooks at the same time?
If you make two different partitions in the external HD, you can easily have the two different Time Machine backup in the each partition.
You only need to specify the partition to use for the MacBook during the initial Time Machine setup. Then the Mac would know which partition to use for its backup the next time you connect the external HD. The same goes for the second MacBook.
By doing so you could limit the amount of space taken up by he Time Machines of both Macs.
For what it's worth, I have this setup running in a few locations with a single NAS, and multiple machines backing up to it. As long as you specify a separate folder, there is no need to create separate partitions. The backups will be completely separated out, and allow you to connect again, authenticate to the folder, and restore from the backups as needed.
I do have separate user accounts on the NAS, so the network folder is logged into by each machine separately with it's respective authorized user.