This takes a moment longer than a regular boot, but it may work better for both of you.
Determine a default OS, and set that via Startup Disk. Whomever uses this OS can boot as normal.
The other user can startup with the "Option" key depressed. After a moment you can select any installed System to direct boot. This is independant of what is set in the Startup Disk control panel.
If a reboot is necessary (say, after installing SW) and the Mac is set to boot the other OS via Startup Disk, then the logged in user can use the Startup Disk control panel to reset the default OS, which will be faster on reboot than the Option key method.
Hopefully the user will remember to reset the default. If this is too much to ask, then just instruct them to use the Option key all the time.