Maybe this is reading WAY too much into a situation, but here goes: Two young people want to cohabitate but one of them is not responsible enought to pay back student loans despite the ability to do such.

You can make the strongest wage earner the primary tenant, the person to stay if relationships fail.If I were you I would change my personal income standard and look at maybe adding other standards that reduce your risk of tenants not following through, such as looking at how long they have had their jobs and how long they have co-habitated.

Local cities have rules on the number of non related people that can cohabitate. 

Most traditional banks (Chase,Wells Fargo) only allow you to possess 4 conventional loans at a time and you must have the income to prove you can pay all 4 payments.After that you need a portfolio lender to continue adding rental properties at higher interest rates normally depending on your credit history and ability to keep full occupancy.I am not a mortgage lender and don't know about how your "spouse" affects your ability to borrow since you are not legally married.You really should have a business contract drawn between yourselves if you are cohabitating and doing real estate and other financial business together.

Then you could do the community cohabitation thing to some degree.There doesn't seem to be anything of importance there like a college to major business that could use stays of 30-180 days.

The 1/1 units were the most profitable overall in high rental price areas as that whole roommate thing has been replaced by "cohabitating." 

Where I am, in the NYC and vicinity, many towns and cities have rooming house regulations where three or more unrelated people co-habiting is considered a rooming house and not allowed.

But having so many kitchens and bathrooms defeats the goal of low cost.I think the most sensible thing to do in order to reduce cost by cohabitating is to split the house into a main part and an accessory dwelling unit, and have two kitchens total shared by five people.

Some municipalities even restrict the number of unrelated people who can cohabitate in a single unit. 

I have lived here for quite some time and most of the homes here are rented "by bedroom" (Possibly one lease but its usually a roommate cohabited venture).