That being the case, it's going to be simpler and more effective to have the fore wing handle both pitch and roll. Fewer parts and less twisting force on the fuselage as then both the fore and aft wings will be contributing to rolling the plane.It can be made to be that way, but then it means that the rear wing is always flying so far away from its maximum CL that you end up with a lot more wing area (and weight, and material, and cost) there than you'd have otherwise. The ideal situation has the main wing, under the worst-case situation, reaching just below its maximum CL capability when the canard lets go and stalls. That results in the smallest, lightest, cheapest design possible for the particular configuration. That's hard to do when part of the rear wing has flaps "up" and the rest has flaps "zero".
It's just far easier to resolve the stalling issues and keep the airplane as small and light as possible if the front wing contains the pitch control and the rear wing has roll controls only, if anything at all. All the Rutan designs, all the tandem wing designs, have done it this way.