You will only be able to count a portion of your rental income, for the unit already rented. Typically banks will not speculate what you could possibly get for the unit you are currently living in so the best scenario is to ensure you have enough income to not need your own unit rented to qualify for the second mortgage.
We now have 4 mortgages and as first time landlords starting out, we did not get any "credit" for the incoming rental income to qualify for our loan, however the appraisal did include an extensive rental section to ensure that the rents would cover the mortgage.
It works best to have a good income stream for the build up phase of your rentals if you plan to get mortgages to purchase them. Good luck!