If you just want to reserve the T1 purely for failover, perhaps the easiest solution (outbound) would be to switch from GLBP to HSRP. (The router with the 20 Mbps circuit being the default gateway and with a preempt.)
If you want to continue to use the T1 proportionally with the 20 Mbps link (outbound), perhaps OER or PfR would do the trick.