I was trying but got an error.. and I don't know if it's possible.Sure you can do that to try it out. It'll work fine.
So no error when install High Sierra on external drive with Sierra on the main machine?Hello, High Sierra on an external SSD is my exact setup. it's on a 250 g Samsung. Fast and furious, Also I have Sierra and Mojave and El Capitan on another external SSD. Mountain Lion (yes) is still installed on the internal Fusion Drive of my late 2012 27 iMac. I assure you there's no problem at all. As far as it goes for me, the fastest boot and performances go to Mountain Lion, El Capitan, Sierra, High Sierra and Mojave in that order. Strange but not so much.
It doesn’t matter what’s on the main drive. You can always install and run macOS on an external drive. What is the error you see?
Edit: You didn‘t say what machine. Is this a T2 Mac?