The axle looks good, but when I transferred the duals from the Kong axle onto my axle... the wheels sorta fell off the bus. When I tightened them down they wouldn't turn. I took things back apart and started measuring bits and pieces.
The 12mm hex driver from the Kong axles is 6.5mm thick and uses a drive pin with a 1.5mm OD. The WLToys hex driver is 5.5mm thick and uses a drive pin with a 1mm OD. I backed things off enough that the wheels turned freely and did a sanity check against the Bruder tires.
From this angle things look OK, however...
From this angle you can see they're too narrow by a couple of mm.
If I make the driver pockets 5.5mm deep instead of their current 6.5mm depth then things should be just about right. I think I'll make the pockets 5mm deep though, because a little clearance nerver hurts Clarance.