Or maybe you want to do a combination of the above?

Real Estate has made MORE millionaires from average John/Jane Does than EVERY other "investment" alternative there is, COMBINED. 

When I combine the leverage with the value add, it is what has allowed me to produce infinite return on virtually all my RE investments.  

Secondly, to keep your low first position rate, there are closed ended second position loans for you to tap into your equity. these loans are fixed terms, usually 20-30 years, at higher rates than your first but when you calculate the blended rate, the combination of first and second rates, are usually lower than a full refinance.

It's about a dozen, identical one bedroom "modern" cabins that sleep 6.

Or maybe you want to do a combination of the above?

Are you looking to combine both your first mortgage on a rental property you own with a purchase of a new rental property where you are assuming the note? 

I want to create a duplex up unit by combining the second floor and 3rd floor into one unit.

We have a modern bathroom vanity setup, but the tub/shower isn't.

Combining primary mortgages (on a subject acquisition) with a fixed second mortgage (against another property) can reduce the loan to value (LTV) on the qualifying mortgage; which should provide lower interest rates and make the offer to purchase more competitive (since there is a larger down payment.)