If I refer a friend to full tilt to get $100 in refer-a-friend bonus, and he fails to gain 500 FTPs, then do I have to gain an additional 1000 FTPs to get $100 for the next friend I refer?

Maybe put more simple: If I refer 2 friends, do I have to gain 1000 FTPs or 2000 FTPs to collect both bonuses?