At the LA Abilities Expo last month, we saw many wheelchair-using kids and adults with one foot on top of the other in a single platform or "foot box" (instead of two footplates) -- a sign of positioning problems for the entire body, not just the feet.
When the feet overlap, the legs and hips can't be both straight and centered. So a lot of the work of keeping someone in the chair is going to end up in the torso (the chest down to the top of the legs).
When the hips are out of alignment, the abdomen and upper body generally can't be well-supported, either, since the legs are not doing their full job.
And then we get to the head and neck. If the neck is struggling to align the head, because the rest of the body isn't bearing its share of the weight, how comfortable can someone be?
For many users, the single platform or box alone works fine. But when the feet are touching and not parallel to each other, at least two things should be considered: ankle support that attaches to the foot rest, such as our Ankle Huggers® provide, and/or dual footplates (yes, we make very nice slotted footplates, too). If you are sliding out of your chair, have spasms or your body arches forward, a leg harness may help as well.
If the ankle support on a single platform can keep the feet comfortable, centered and parallel, the platform may be enough. But if not, it may be time to consider each foot's having its own platform, allowing for separate adjustment as may be needed since two feet may have different positioning needs.