Two Macs, one external boot drive (not at same time)

I'm in a weird situation where I travel b/t two cities during the week and am looking to have a computer in each. Have been doing the laptop thing, but it's too heavy to drag everyday and too slow for the video editing (I'm a filmmaker) I need to do now. So, I have a 1TB SSD and am looking to make it a boot drive (via USB 3 or Thunderbolt) for two different Macs. Right now I'm looking at two iMacs, but I might want to make one of them something else, possibly a new or older Mac Pro.
So, would I be able to run both Macs with one external drive? Also, would I have any issues if I go with two different kinds of Macs? Thought maybe there would be an advantage to having the same with connection issues of some sort.
Thanks for any help!

Let me ask: how are you getting your two OS's on the external drive?
For one thing, if I understand you correctly, you want one OS on the external and use it on both Macs? That, according to the licensing agreement, is not allowed (see excerpt below); you're allowed one backup copy officially. And, secondly, you cannot boot a Mac with an OS version earlier than what it came with, so it would depend on the build of your Macs (and the exact build version it came with). My initial thought was that you clone your internal hard drives to the two partitions on the external and run them separately.
For now, just a guess on your Q #1 - not sure, depends on where it originated (see above for not booting with an earlier version) and, I prefer cloning FROM the internal system TO the external (again, because of build versions/firmware).
Q #2: see above re. software licensing agreement.
Q #3: not sure at this point
As for the SLA - if the OS is preinstalled, the license is for that computer only:
Here is a link to the SLA:
And, FWIW: no, once you mark an answer, it cannot be undone - hopefully, that won't matter and someone else will pop in with a brilliant idea.....

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