Two people renting one property how do I make sure rental continues if one moves out?

Hi. my mom has an apartment rented out 0 she usually rented it out to a family but this time around she is renting it out to two friends. They seem sound and lived together before (not a couple) but my concern would be that if one of them decided to move out for whatever life reason, then the other will have to move out took. I want to build it into a contract that they are both individually responsible so if one decides to move then the other will have to continue with the rent. Is it possible? What is the legal term for it? Or should I just make a contract in one of names only (they don't mind)?

Just looking at the standard contract that mom has been using previously and there is this provision: “The Tenant” includes the successors in title. Whenever there is more than one Tenant, each and every covenant and obligation can be enforced against all the tenants jointly and against each individually.
Does this cover us then? or is there a more specific language that is needed? CAn anyone give an example?

Is the lease not signed by both the tenants?