Best Way to do Joint Co-Borrower Conventional Loan?

[Daniel Dietz]

Hello, 
I am looking for some advice/clarification on how conventional (meaning 30 year fixed) loans work for rentals when using a partner/co-borrower.

A couple specific things are;

1) How does joint DTI work? Meaning if I have say 1500 in monthly debts and 3000 of income mine would be 50%. If me partner has 1000 of debt and 5000 of income theirs would be 20%. I assume our joint would be 2500/9000 or 28%?

2) Does it matter how long our joint account has been open as long as the funds can be traced back to one or the others individual bank accounts?

3) If we have a rental property where 75% of rents cover PITI, does that mean that neither of our DTIs would be negatively affected as I understand it?

Thanks, Dan Dietz