Hi Dave
Ulster Bank is the only lender which will allow you to keep your tracker while trading up.
They probably won't do this for negative equity, but if you are in deep negative equity, you probably should be focussing on clearing that first, before borrowing more money.
If your savings clear your negative equity, you should have a chat with UB and you may come to a deal with them. A reasonable deal would be to have you to clear your negative equity with your savings and then they will lend you the additional money you need to buy a bigger property at their SVR.
Overall, I think it's a bad idea to combine houses. The new Local Property Tax is based on the value of the property, so two houses will cost roughly twice as much as one. Water may be cheaper, but not if you are metered. The insurance company may even charge you some form of additional premium for combining two houses.
Brendan