A few existing homes in the community are $70k annual. 

Essentially my idea would be to sell 1/2 my interest, pay off the existing loan, and be "partners" with this person as long as there is a defined exit strategy. 

Do they exist and where is best to look?

Real estate syndications have existed for decades, with some of the earliest examples dating back to 1926.

@Roger Verastegui I’m just starting to poke my nose into new home builds on existing lots in he Seattle area as well.

It may not be all of them, but they exist.

I could pull a heloc on my existing primary home (in Florida) to purchase the land and use towards the build along with cash (50/50).

I haven't seen anything under 200-250k all in cost  Part of it is detached garage and using existing structure + existing electrical.

Generic agreements/hand shake agreements with vague exist strategy can get a little nasty in my experience.

If you decide to keep the home, you could refinance (and lose the 3.5% rate) or get a second position loan - this would allow you to access the equity without affecting your existing mortgage.