I'm trying to buy two different packs, the Dungeons and Dragons Desire and Deep in the Dungeons. However, it seems I have to purchase them separately. Since I have a limit on how many times I can use my bank account, I'd prefer to buy them at once. How do I go about doing this, if it's possible?

I just ran into the same problem, but I don't have any limits, so I just went through checkout twice.

My guess is that with a promo pack which has a discount applied, you can't add anything else to that particular order, or the discount would apply to that too.

At any rate, promos don't get added to the cart.

Add all games to the cart, discounts will apply. Not sure if that counts the surprise boxes, but for the other bundles it works. If you add the individual titles, you can buy all the promos in one go. Has been that way for quite some time.

Hmm, what about games that are in different bundles with different discounts?

EDIT: For instance Carmageddon 1+2.

Not sure. I think it will give the biggest discount it is available in. So if the recommended is 66% and the community is 75% if completing the bundle, you will get 66% if you don't complete the recommended or 75% if you have everything else from the recommended.

If I check a single game from a promo and go to "checkout now", then go back to the front page to add another, my cart is empty.

Don't go to cart from the promo page. Open the individual game pages and add the games to the cart. Then go to checkout as you normally would.

This is what I do to add all promos to cart.

Open each promo's page that you want to buy, in its own tab, he urls look something like this:
http://www.gog.com/promo/name_of_promo (don't click 'complete set' or 'buy now' it only works from the promo urls)
Click "gift entire pack" then click "Yes, gift this pack".
Now you are on that promos check out page, click the little X to delete the games you own/don't want.
Now that you have the games you want in your cart, close that tab.
Now if you refresh any gog page, your cart icon on top of page SHOULD now contain the games.
Do this for each promo you want, then JUST before you pay, uncheck gift, or leave it as gift.

I tried adding the games individually, and it only gave me the discount for one of the bundle promos. I ended up buying the bundles separately in the end. Not that it was a big deal to begin with, but still.

Thanks a ton, any ways.