In order to have two-tier affiliate revenue sharing, your affiliates need to have revenue sharing agreements with people who sign up from their web site. You would need to pay the 2nd tier affiliates the normal commission and the 1st tier (who initially signed up the 2nd tier) a small commission for each sale the 2nd tier affiliates make. This is basically multi-level marketing (MLM).
Strategically this method works because each affiliate has incentive to recruit more affiliates for you. The tracking is a bit more complicated than a straight-forward, single tier affiliate program but the benefits of this method can be profitable.