It would have a different account reference. I think KBC do a sub reference eg. Account 345678 - 2 loans 345678/1 & 345678/2.This would be an interesting discussion in its own right - is it treated as similar to a split mortgage? Does it have its own mortgage account? Can you have two different fixed rates & periods on it? I have never done a top-up, but would be interested in how clean the banks are in differentiating between the two.
I think "Top-Up" is just a user friendly way of describing it, but in reality its a separate loan but the security of the property is applied to it.