is there a way to run Orbot and Mullvad side by side on Graphene?
Discussion
Unfortunately not, they both use VPN
They're both VPN services, and you can only have one running at a time.
Tried myself. Did not work.
Yes you can run power user mode which runs tor as a proxy on port :9050
I'm not sure if the hidden services work still without the VPN function though. Will require testing 
"Multiple users" have different vpn instances, but for any single user on gos, one vpn at a time. I wish split tunneling was possible without risking dns leak, but no Christmas in October here...
If I have a spare minute, will try testing power user suggestion.
I have been able to run tor through my vpn, but not in parallel. For me, running tor through vpn gets the functionality I need.
How did you do that? Power user mode?
Yes
An alternative that I haven't tested but am 95% sure will work is (using GrapheneOS)
1. Enable multiple accounts
2. Create a fresh account with a tor VPN (orbot)
3. Create the service you want to access via onion. Run the service.
4. Switch to main account, but leave the new account running. You should now be able to call the onion service from your main account.
I haven't tested this, and I am not sure if this is useful to anyone, but just in case...