Where did you hear that "stacking" will be some new kind of gameplay element. Wasn't it just to save space on rows and stack copies of same units on top of each other. What are you talking about.
It would make sense though. To stack on a card when running out of space, with the card(s) underneath not adding to your total score, or something like that. But yeah, first of all it's not really a mechanic that makes removing 2 rows worth it. And second, could be done with 3 rows, even in current Gwent. Place cards over existing cards, but lose power (and any abilities of course) of cards you're covering.
This is actually a great idea. You cover cards denying their abilities, losing tempo, but at the same time protect them, to maybe at a later time move the card on top, reactivating previously covered card's power and ability, gaining tempo. But did you notice the important keyword, "move". Movement would of course work a lot better in a 3 rows Gwent. If you can just cover any cards like that at any time, how does this exactly work better in 2-2 Gwent.
But let's say you can only do that if you reach a limit, fine, maybe make 3 rows Gwent's row limit 8 or 7, then you would see this mechanic in that system as well. But it's a bit weird if it would only work when you reached row's limit. It's a mechanic that should work all the time, because it loses tempo, and the card underneath "freezes", if it had any abilities.
The first time I heard about something similar was from a reddit thread a day ago I believe. But now I can see how it could work.
The unused damage dealt to the card on top would of course go through to the card on the bottom (not effects like scorch though obviously). So this is how "Trample" could work in Gwent, and removal cards would more often find more value. (I don't want stronger removal, there will be a "Deflective" mechanic as well to some cards, that counters bronze and silver removal, doing only half of its damage to target unit, with the rest of the damage deflecting to another unit).
There is an issue with the benefit it actually gives. The way I described it, it would give benefit only in specific situations, and if you really want to put all your units on one or two rows. If this was to become an additional archetype, there could be interactions like boosting the unit underneath when placing one on top for a later profit. Adding tags/items to units underneath that would maybe be too strong if added on an active card. There are certainly some (probably many) possibilities here.