Shared house

Tenancy Types and Management

I have house which I hva let it out to 2 couples and 2 individuals.
I have a AST agreement on stadard form Agree20/1 by Oyez with all their names and signatures on the same form.
I now want to change this to individual(or a couple) AST agreement,i.e have a separate AST agreement for each room and common
areas. However, the tenants should still share the utility bills.
I think this type of agreements are called licenses. But then will these qualify as assured shorthold tenancy agreements?
At the moment the gas bill is in one tenant's name and the electricity bill in another tenant's name. As far as possible I want this arrangement to continue.
Could you suggest an agreement form for such a arrangement, please!