hi.
Once I do it but with e1s in serial interfaces, Try PPP MULTILINK.
Obviusly In my case there was two clear channels, I changed the encapsulation to PPP, and created a multilink interface, its a virtual interface where you attach a bundle of physical interfaces where the frames are fragmented and balanced over the physical interfaces associated to the logical (multilink) interface.
try searching for multilink.
luck.
Its the same concept in ethernet like ether channel, but with serial interfaces.