You can look at the lease. Make sure it says "jointly and individually responsible". That means if one moves out, the remaining parties still must pay the rent. I'm not sure what the best way to remind them of this clause is. Also, search for a "Roommate Addendum", which will help you move in one roommate when the other moves out.
I don't know if you should get involved in their matters, so long as they pay rent on time and don't damage the property. Looking forward to hearing from others.