Backface is an object property, so simply assign the material the regular way (and it will be applied to the front face) and on the object's Maxwell properties panel, there assign your other material for the back face:
I'm doing this right now by opening final scene in Maxwell Studio, applying back face materials and starting a render from there, but I was looking for a way doing this in modo.
I'm wondering why there is no back face material setting in object properties in modo plugin...