Bonded Interfaces operate at a lower level. There are many different "bonding modes" in the Linux Kernel. None of them would allow you to present a single "bonded" interface across multiple independent network interfaces where each interface belongs to a separate network. Case in point: You cannot "bond" two interfaces where one is connected to a cable modem and the other a DSL modem. Typically (in most configurations) a bonded interface receives one IP.
You can, but you need to make two tunnels (GRE, IPIP or whatever you fancy) over those two uplinks, then create a bond over those - if that's technically possible (I believe interfaces must at least have equal MTUs, can't remember what other requirements are). Obviously, this requires configuration on both sides of the bond.
And, ahem, correct load balancing over bonded interfaces is not trivial as it may seem (although, usually it's more likely you'll be able to distribute approximately evenly than not)
