!!! WILL NOT WORK ON 64-BIT OS !!!
The first one is like the GOG one I think, an adapted GOG version like the Master anthology. The question here I guess is if the Other DRM because of the second has changed since they released that one (on GOG) as well now (after they put the packs on GG). Possibly they have, or possibly they are still selling out old keys, or possibly they simply haven't bothered to change the deal.
The 64-bit OS is a general warning but has generally been found to be nonsense, pretty sure my Steam version used to say the same and that one worked fine on 64-bit os. Some replies on the GG page are saying the same.
NWN alone on GoG is 10$
but if NVN2 will be on discount again will consider paying for it
His point there is that the $8.99 Neverwinter Nights Complete you linked includes both NWN 1 and NWN2. Think you may have gotten that already but it wasn't clear from your post.