Cooling capability is directly related to radiator surface area. Why not simply allow for thicker (>30mm) radiator(s) on one side instead of a second 240mm unit on the opposite side of the case? Anyone wanting to cool both CPU and GPU on separate loops could, for instance, opt for two 45mm thick 120 rads.
Such an arrangement would still benefit from the efficiency of directly-mounted fans, and radiator volume would grow by only 50% vs. a full doubling in the twin-240 configuration.
I'm no expert but I was under the impression that radiators are more effective per unit of volume when that volume is spread out over a larger area, as opposed to short "stacks". I imagine having a thick enough 120 rad to compete with an equivalent [slim] 240 rad will increase the resistance airflow through the rad (which would need to be 60mm thick). Longer, slimmer rads out-perform shorter, fatter ones unless you really crank up the RPM, at which point you'll have ruined your acoustics. You might shave of 50% of the rad volume (if you use less than 60mmX120) but my napkin math tells me the loss of surface are would exceed 50%, all the while adding resistance to the airflow. I also believe that as air passes through thicker rads it becomes saturated with heat before it can even exit the other side, further reducing the efficiency. I guess the main caveat to this would be that your idea allows for direct placement of the fans, which may assist in overcoming the added resistance. We'd need to see numbers. I don't have enough napkins to do it all in theory.