Since 870s do support vlans, it would be possible to treat each ethernet port as a single port (and as a wan port).
I see here two solutions for load balancing/sharing:
With both solutions, you will establish two path to a destination device (in the SP network) that will aggregate the flows.
With CEF, two IP (L3) routes are established over the two modems to a router that has your two next-hops on its interfaces.
With MLPPP, two ppp tunnels (L2) are established to a LNS. That would require to terminate the ppp session on the router and not on the modems (also allowing the possibility to load-balance with the onboard xdsl chipset).
Load algorithms are different, and I would advise you to read the documentation of both methods to see which solution fits you best (some kind traffic is much more sensible to out-of-order packets than others).