He also promises very low vacancy, two to three weeks between renters, and they eat the costs if they get a bad one.

Turnover costs can eat you up with property management taking a months rent for placement.

Plus with 1/1's being a lot less stable, prepare for your property manager to eat you alive when replacing tenants.

I think you are aggressive with how many and a retirement date but I think if you eat sleep and breathe it it will happen.

It sounds like you have a solid plan in place and realize how much HOA fees can eat returns, a house hack may indeed suit you well even if it means a longer commute.

Should I send half the bill to each or eat the cost.

Be careful with mortgage insurance, which can eat away at your cash flow.

They are ordinary income and you've likely got a bit of a struggle on your hands to work this through.You can absolutely set up retirement plans based on your flipping income, even if you're flipping with partners, but you're in a situation where you're trying to have your cake and eat it too.If you are really going to insist that your flipping is capital gains (it's not, but let's say it is), then no - no retirement contributions because retirement contributions are based on EARNED income and capital gains is PASSIVE/INVESTMENT income. 

That said if it gives me more freedom to make the right career choices or it allows me to act on investment opportunities in the next couple years, maybe it will have been worthing eating additional expenses for the first several months.  

Hopefully they at least buy a few as vacancy would mean you don’t eat that month!