Having the seller carry a second is not a problem for the first, as long as their business with you is finished and the loan has closed.

House hack another property with a low down payment loan and use the remainder or the proceeds to either renovate or purchase another investment property?  

You can also see total loan exposure, every loan ever pulled, every property ever owed, every co-signing partner from every project, permits, etc... 

If I got a home equity loan for Property 1 and used the proceeds as downpayment for Property 2 and then got a 2nd loan for the remaining 75% purchase price for Property 2, when I sell Property 2, can I use the Property 1 loan as part of the cost basis for computing Property 2 capital gains?

The idea is to use this property as an office space, giving me the separation I need from home while providing a place for my assistant to work where I can be more hands-on with them.The business would pay rent for the property, which would cover its expenses.

If you're confident you can find better cash-flowing deals or scale your portfolio with leverage, it could make more sense to reinvest rather than pay off the loan.

I doubt he'll be able to quality for a loan (VA) with just his income. 

If they are the one providing most of the cash, they might not want to carry that liability, but you've already negotiated with your partners on this and then the bank isn't cool with it, you could be in a pickle.

Ideally you can provide something extra or unique that you can highlight in your photos and description to stand out. 

You could use a HELOC, but with VA loans you have an allotment so you can get loans on more than one home depending on how much bandwidth they give you.