I think you are making the folowing mistake, all provided I understood correctly.
"Adding" the ice heightfield to the other is not what you want. This is why: Let's say a given point on your landscape is 100 m high. The height of the same point - but on the ice heightfield - is 102m high. If you ask TG2 to add the two heightfields, the result will be a point that is 100+102=202m high.
Instead, you want (using the hieghtfield merge shader) have TG2 choose the highest from those two heightfields, at any point. You would have to set "Merge mode" to "highest" to get that result.
Let us know if that works for you.
As a side remark I would like to say that I don't believe this will be a good method for realizing your idea. Along the edges of your "ice mask", you will have pretty steep slopes for the ice, I would guess.
Regards,
Frank