T1 bonding

Can someone shed some technical light on the details of how two T1's are bonded (typically). We've got two sets of T's at two different location with vendor 'X' (name starts w/ an 'A') and it appears that we're really only getting about 1 full T's worth of bandwidth and maybe 20% of the second. Seems like they're bonded perhaps using destination IP? It's a vendor managed solution and I need to get some answers faster than they're coming in. Thanks. Matt