If spouses file a joint return, the joint venture between spouse is a qualified joint venture that is not treated as a partnership. 

My parents invested their HELOC  in a restaurant as a joint venture.

Have a Joint Venture Agreement that outlines who puts in how much money, how disbursements are made, what happens in the event one of you wants to leave the agreement.

I think we are in the 15% bracket (married filed jointly with income between 70K and 400K) for Capital gain.

I take on investors as Joint Venture Partners and we put Facebook to shame!!!

If that's the case, in the future you might do as @Amy Beth suggested and make it so all tenants are "jointly and severally liable" for the total rent. 

If that would not work, would it work for his SDIRA to put 10% down, my non retirement LLC to put 10% down, and then jointly borrow the other 80% together? 

To me that extra step is worth not having a balloon payment and slightly higher rate.Now with joint ventures, properties that are 5+ units, or it is short term financing I look at hard money, private loans, commercial financing, etc.

@Miguel Castillo Miguel going to challenge your KC food knowledge here... ask your network for the best vegan joint in town!

I track all of my spending and deposit exactly what i need to pay my bills into a separate joint account, all of my bills deduct from that account and I give myself a strict food and gas budget for the week and thats really it.