I think you could do it like so:
- copy your entire bannerlord folder in place, it will take a while of course, and will create a "Mount & Blade II Bannerlord - copy" folder, replace the "- copy" with "main" or anything to let you know that it's the main branch version.
- now go to steam, right click bannerlord in your library > properties > betas tab and pick the 1.1.0, the game will download some 2 or 3 GB worth of updates
now whenever you want to switch from main to beta you just rename the folders and change the beta option on steam of course, so if you're in beta but want to switch to main you'd rename the current bannerlord folder to "Mount & Blade II Bannerlord - beta" and the one that has "- main" to just "Mount & Blade II Bannerlord", plus go on steam and opt-out of the beta, and vice versa.
I haven't tried this myself, but I'm pretty sure it would work.