There is no real issue with how many mortgages you can hold however there will be big issues depending on the TYPE of mortgages that you hold.
You can only legally hold 1 (one) residential mortgage as this is a mortgage designed to be on your main residence and as you can only have one main residence you will not be permitted to hold more than one residential mortgage.
If your other property is indeed a pied a terre / second residence / holiday home then if this is disclosed to the lender then they should allow you to take a new main residential mortgage on another property as long as the new main prperty is not within a certain distance of the pied a terre....normally 75 miles.
If you still run into problems then you can look down the route of consent to lease or buy to let.You do not have to lease the propert but it may just free up affordability and more inportantly remove the pied a trre from being counted as your main residence. If you are currently tied till 2013 then it would be wiser to look towards the consent to lease than a newbuy to let product.
As Maxsteam has suggested you should look to speak to a few mortgge brokers to get specific advice about how to move forward and who to move forward with.