2 Keys, 1 Copy?

Have that with the Duke Nukem collection, i tried several ways, i just now give them when the giveaway ends.

yeah, no.
the system would break down. there's too many "ifs" that'd have to be covered and I bet ya 90% of ppl wouln't give a thing about any instructions, would paste the keys whatever, f everything up and then come crying at the support adding to the butt-load of tickets they already cannot resolve...
it's pretty niche to have multiple keys you'd want to give to single winner and thus implementing such a system is too much work for what it's worth.
your best bet is to simply contact the winner and send them the DLC key separetly, or just make a second GA just for the DLC and rake extra CV for it ¯_(ツ)_/¯

the two keys (game and dlc)

Why can't you just make a separate giveaway for the DLC on its own? It showed up on the giveaway list when I looked for it.

because I clicked on 1 copy, entered the two keys (game and dlc) and didn't notice until after creation that the system interpreted that as a multiGA, disregarding my prior click on 1 copy.

Site automatically pick gift and 1 copy as the default. It's not possible to input number of copies in case of keys, site detect them automatically, which IMO is a nice feature.

If the extra bundles ccan not be added, could we maybe have a feature that makes multiple keys per copy possible?

That would just make people keep picking the wrong option and using default "2 keys 2 copies" when they have game + DLC, or selecting game + DLC when they have two keys for deluxe edition, which would net them only CV for 1 copy.

Simple solution is to send winner game, confirm they received base game and then modify key for DLC. Tell them they should reveal key again and activate DLC.

You can just use the gift option and then email them both keys instead

It seems not. You will have to send the DLC key to him separately.

just send them to email.