Heya 
It’s possible, but you’ll need to pay careful attention to your installed packages (modules/systems) when you switch between your different games.
Your two games, one on v9 and one on v10, will be separate worlds so they won’t touch each other and you can have both at different Foundry versions. But the packages need to change every time you switch between them.
If there is no overlap between the packages you use between the two worlds, then you can simply switch the Foundry server version and Stop and Start your server when you want to switch between games.
If there is overlap, eg. if they both use the dnd5e system, you’ll want to follow these steps every time you switch between Foundry versions.
- Stop your server
- Change Foundry version
- Visit the the Bazaar and auto update packages
- Start your server and play/prep!
Hope this helps!