Hi Mango
I am Dave, an Independent Advisor, I will help you with this . . .
It is very strange that those PC's have different accounts and yet that change affects both devices, do those PC's share a common Admin account?
On both PC's go to Settings - Accounts - Sync your Settings
Turn off all options on that page, then restart (not shut down) the PC and set the preferred mouse button configuration, then test to see if that behavior stops . . .