I did exactly this with my 2015 iMac. Bought a Samsung T5, cloned my internal Mojave fusion drive to it using SuperDuper, booted from the T5, and allowed Apple to install Big Sur on the T5. As far as I can tell it had no effect on the internal drive, and I can now boot from either one. I was careful to disconnect or turn off any backup drives/software so that there wouldn’t be any confusion about what drive was backing up to where. The T5 feels much quicker all around.
My goal was to see if any software I use was impacted by Big Sur. It all seems to work as expected but I’m still evaluating Big Sur so for day to day I’m using Mojave and have my normal 3 layered backup system operating on that.
There have been some issues with signing in to iCloud when switching boot drives and sometimes this caused problems with other synced Apple devices, but not a big deal. I’m basically waiting to see if SuperDuper/Apple will be able to do data backups from Big Sur before switching to that OS.