Can shares be unequal?
Yes — any split that adds up to 100%. Equal shares are the default.
From the guide Splitting Rent with Roommates.
Changing or ending a split
The roommates and their shares are fixed when the lease is signed. To change them, the landlord ends the split and sets a new one up on an amended lease. When a split ends, the full rent is charged to the tenant of record from the next unbilled charge, and every roommate is emailed. Ending the split does not release anyone from the lease.
Read the whole guide: Splitting Rent with Roommates