Using current rental income to qualify for a new property

[Luc NA]

Hi, 

I see a lot of information about using future rental income to qualify for a 2-4 unit property. However I cant find anything online about correctly factoring in current income properties into a future property purchase.

I currently live in a duplex and rent out half of it. I would like to purchase an additional property, however my existing mortgage makes that a bit more difficult. 

I have been told sometimes that I can add my rental income to my total income to help qualify for a new property, which helps somewhat.

I have ALSO been told that I can subtract my rental income from my monthly mortgage expenses, more like a business. That would obviously help a lot more by reducing my monthly debts (for debt to income guidelines). I have not been able to substantiate that claim. 

Can anyone help me on this? What is the best way to go about qualifying for a second property?

Thanks in advance for any tips!