Can you count income from first o.o. property to qualify for 2nd?

[Sarah Lorenz]

I'd like to help relatives with a moderate income buy a duplex in order to live rent-free, probably with a Home Ready mortgage that allows the rent in the 2nd unit to count toward qualifying. I'm wondering if they could then purchase another property to move into later on--can they count the income from both units? Would they have to wait a certain length of time or factor in anything else?