How to giveaway Frozen Synapse prime double pack

Don't know if it's allowed to make a "I'll send the gift to the first winner and he's required to send the additional copy to the second winner" giveaway. If it is it's still risky since you don't know if the winners will cooperate.

If it's allowed I would make it a higher level giveaway, that way chances that some asshole will run away with the second copy are lower then when it's level 0

can't be done. one key/gift, one redeem. only the "redeemer" will receive the extra copy.
in this case, if you can only send one key or one gift (even with extra copy), don't assign two giveaways.
you can ask to winner to return the gift but it is up to winner (as hagehage said).

I think literally every Steam copy of Frozen Synapse Prime comes with that gift copy, so the "double pack" doesn't really exist. (I got two copies from a bundle key, and the Steam store page advertises that the standard package comes with an extra gift.) I'd say give the key to a friend you trust, then have him gift the extra copy back to you to give away on SG to get the most for your money.

you can ask users entering your ga to give you back the gift after activating the key. it would be common courtesy on their part.

of course, this is up to the winner since you can't force them (you can blacklist him if he doesn't do it).

or make a whitelist ga and ask in the forums who would want to enter with that requirement.

When I made GA like that, I always asked the winner to giveaway extra gift(no trading) for other people's chance. In most cases because they were Group GA, the things went well as I intended.