"Once you spent 5.00 with Green Man Gaming you'll receive your own referral link.
Share this link with your friends and when they start buying, you'll start earning. For every friend you refer who spends a minimum of $5.00, we will credit both your accounts $5.00. If you've got lots of mates, you could be quids in!"
Here's how you use this referral program to get both for 10 dollars.
Step 1: Find a friend with a different IP than yours who doesn't have a Green Man Gaming account (I'm pretty sure if he has the same IP, they'll recognize him as you and the referral won't count).
Step 2: Spend 5 dollars on your account Green Man Gaming. For the purposes of our goal, you'll buy Just Cause 2 (note: Lara Croft won't trigger this because it's under 5 dollars).
Step 3: This will unlock your referral link; give it to your friend. They will sign up for an account there.
Step 4: Using this account, your friend buys Just Cause 2.
Step 5: You're both credited with 5 dollars, which you can then use to buy Lara Croft and the Guardian of Light.
If you've already spent money there, you can change this process up by requiring no initial purchase. You also don't have to buy these games in particular. There's a spring sale going on right now that you can use along with this referral mechanism.
Both games can be activated on steam, by the way.