Grargar: Unfortunately, no. You can only use each code for a single $5.99 game, thus totaling two $5.99 games. GOG has no wallet to speak of for the time being.
Pheace: Are you sure it's 2x 5.99? What's the EU price for the game?
It's $5.99.
I live in Germany, and because I already own W1 and W2 I had 27% discount. So I payed 43.79 EUR. Then the Fair Price Package:
Since the price in your region is almost $11.98 higher than in the US, we’re giving you $11.98 back (~€8.98) in bonus codes to get free classic games from GOG.com.
This resulted in 2 codes of 5.99 $ each.
Pheace: Are you sure it's 2x 5.99? What's the EU price for the game?
Grargar: I'm not sure at all. :P
He says:
"If I have 2 codes, each worth 5.99, can I combine them to buy a game for 9.99?"
And, thus, I presume them to be each a $5.99 code.
Edit: But come to think of it, this is correct. Assuming he owns both Witcher games already, he pays a total of €44 and gets 2 $6 codes. Thus, about $65.
9.99 was just an example. I could have said 10.99, or 11.98....
2x5.99 = 11.98