Isn't that 6v instead of 3 though
I'd agree with that, these "coin" cells have the contacts on both sides - and any keys I've seen have the lower contact at the bottom of the battery "well" and the higher one is at the side, so using two cells stacked one on top of the other would probably end up shorting out the top cell. I have seen the older
Ibiza 6K remotes and they have a different contact arrangement and strangely use a spacer between the cells, also my B5 Passat might have two "thin" cells - but the Polo 9N has a single "thick" one.