The answer to your question is dependent on whether or not your debt to income ratio qualifies with both the current mortgage payment and the mortgage payment on your new home.
A common question is whether you can rent out your current home and use the estimated rents received as income. In order to use expected rental income you need to have sufficient equity in your home (25% - 30%), at least a 12 month lease, proof of first month's rent payment, proof of secuirty deposit, and a savings account that contains 6 months worth of reserves.
The reserve savings requirement would be the amount AFTER the down payment on your new home. Let's say your current home payment (including taxes and insurance) is $1000 and the new home payment (including taxes and insurance) is $2000. Your total monthly housing expense is $3000. 6 months X $3000 = Savings of $18,000.