It's basically a legal 2-unit that can cashflow $900/month.

Or really they were ALL pre foreclosures when it was legal to do this in OR and WA laws changed in 08.

My favorite sport is boxing and I like to run in my spare time.

Any legal issues I should be aware of in this situation? 

Hiring a PM can save you time and headaches by handling tenants, maintenance, and legal stuff.

Perhaps if your partner was a general contractor or handyman type there may be some merit there to reduce costs / increase profitability.If your partner enjoys deal sourcing, a better move might be for him to become an agent - so he could legally collect commissions from finding deals and managing properties for multiple people. 

Signing the lease demonstrates they agree to the terms and then you can hold them accountable.If you don't do this, then the person named on the lease and paying the rent could disappear and you would have no legal authority to charge the remaining three for unpaid rent.

All existing STRs with 2 days rented in the last year will be granted legal non-conforming status.  

Most are fine with that, but a few will have problems with it and claim there are legal issues, etc..

I have never done this just thinking out loudNo legal advice.