Re: Would it be possible for us to trade up?
The negative equity on your home will restrict you from moving that mortgage to another lender but your current lender may be open to switching it to an interest only investment mortgage depending on the rental income it can achieve.
Unfortunately for you, the bank will probably look for the rental income to be 1.5 times the capital and interest repayments. As there are many unoccupied properties out there, some lenders are also discounting the rental income by up to 20% depending on where the property is.
Of course if the property is in a much sought after area then this should not apply to you.
Your salaries are strong enough to support the new mortgage and should also be strong enough to make up any shortfall in the rental income so I would say you have a chance of approval but in this market it is hard to be certain.