A couple X-Wing designs have been posted in the X-Plane groups. X-Plane simulates aerodynamic forces and does a reesonable job of sorting flightworthy shapes from duds.

The best guess I can offer on x-wing planes might be that they do best in high-speed planes that need to manueuver a lot and that depend more on power than on the size of their wing to stay aloft. The reason: Biplane wings of most sorts cause disturbances in the air, especially downwash, that interfere with wings behind and below the top wing. Downwash might be more critical near the middle for several reasons that I wouldn't write about without doing some reasearch first.

A purely perpendicular planform would always want to favor one plane or the other. A more practical geometry places the wings closer to each other, like a tall, leaned-over X. A more practical anhedral/dihedral configuration for lower speed flight puts the wings the other way, so the most downwash is at the tips - more like <>, but with the tips separated a little bit. I think this, <>, is a very nimble, though somewhat less stable than a double dihedral biplane configuration.