It can be a leaky upstairs toilet or tub or shower, a hole in a water pipe in the wall, a water pipe joint that was improperly soldered.I've done flips for 30 years, everything from houses built in 1899 to present.

If a deed doesn't state that the grantees hold title as joint tenants with rights of survivorship, then I believe they hold as tenants in common. 

I would have each of us have our own LLC and do a Joint Venture Agreement with 51/49 ownership and 50/50 profit.

. $500k if married and filing jointly.

I just use a Joint Venture Agreement with distribution of profits that reflects the training and the risks.Here's a post I did on Subject To that may helpUsing Subject To, to Get "Free" Properties https://www.biggerpockets.com/forums/311/topics/1060320-using-subject-to-to-get-free-properties-a-quick-guideline 

Partnerships are messy.I'd suggest you either act as a hard money lender for the project and follow standard underwriting procedures with a DOT & Promissory Note prepared by an attorney or form a Joint Venture where you each have separate entities that are coming together for a common goal.

Should we take out a joint mortgage?

You file seperately or jointly?

My wife and I have a joint bank account, And she also has her own personal account.

They make their real money by massive advertising, & youtube gives them an army of people looking for properties for them to joint venture with.