Themble
Appreciate the idea, was really excited, but unfortunately, it's not working out on my device.
Installed Molly in the target profile, gave it a different name than the install in the other profile, copied the key hash, switched back to the profile with the original install, unlocked Molly, select link another device, pasted the key, got the brief pop up dialogue, linking device - but nothing. Switch back to the linked profile instance, spinning wheel at the Molly screen, that had previously showed the key to be copied. Stuck like that forever.
I don't know if it's because the original Molly instance was key locked, or if it's because it's not in the owner account.
Frankly, it seems stupid to me that, the resident messaging app has unrestricted access to any and all additional profiles, but the same access by a secure messaging app is out of bounds.
GrapheneOS really needs to do better on this point.