Joint ventures on STR or on flips would help you scale 

The quality of modern lumber is simply far inferior to the quality of lumber they were using to build homes in the early and mid-20th century.

We jointly took part in the design phase, and they got what they wanted and I got what I wanted.

Partnering with investors  and also doing flips using a Joint Venture would get you  more capital!!

I have 2 separate Property owners where im located in Maine both looking to do joint venture deals where they provide the land and I get the project lined up and funded.

Depending on what the line looks like (i.e. if it's just a small gap in a joint/imperfection in the line that the roots are growing in, and not that a whole section of the line has collapsed or anything like that) then you can probably try this first before replacing the line.

are they filing tax returns jointly or "married filing separately"?

Most leases make tenants like this "jointly and severally liable" meaning they are BOTH responsible.

Thus, as I am at this point a ‘hobbyist’ and not a professional I presumed I would simply report whatever minimal profit or loss realized on this first joint flip project as pass through income.

Please make sure you vet your partner properly, I wouldn't recommend doing a joint venture or partnership with someone you don't really know.