Cheers mate, you have made the APC60 become a reality. 
This literally took me about half an hour, a philips screwdriver, a hex driver to fit the external apc ear screws and a hacksaw blade. Done.
...I can't wait for the first person who connects up a full six set of apcs this way. Awesome.
OK. First here is the finished article:
And here is step-by-step instructions. (note: you will certainly void your warranties and I am not responsible for any damage you may cause to your units. You follow these instructions at your own risk.)
1. Remove the external hex screws that hold the side ear in place.
2. Remove the x-fader knob on your APC40 by simply pulling it off and set aside.
3. Flip your APC40 over and remove the two long screws that holds the x-fader module in place.
4. Lift out the x-fader module and unclip the cable and set the module aside.
5. Remove the rubber feet, they all have screws underneath. Worry not, they come attached by some re-useable lo-tak gear so gently remove them and set them aside.
6. Remove all the screws and lift the base plate off from front to back as the cables connect at the rear to the board, stand it vertically behind the apc unit.
7. Remove the internal screws that hold the side ear on.
8. Remove the ear.
9. Repeat steps 1,5,6,7 and 8 for your APC 20.
10. Nibble off the plastic fins that stick out on the now exposed plastic sides of the apcs with a pair of pliers.
11. There are two large holes that extend out preventing a snug fit of the apcs, gently saw them off with a hacksaw blade or similar.
12. Marry up the two apcs side by side, keeping them upside down.
13. You'll see the large hole at the back that you trimmed with the hacksaw blade is ideal for your first bolt and then there are two more holes that marry up, one in the middle and one at the front although these are smaller and may need making larger to accomodate your bolt.
14. Feed the three bolts in to place and tighten them paying attention to the flex in the plastic, needs to be solid to hold the apcs together but not too tight that it warps the plastic.
15. Put the bases back on both apcs and screw them back into place. Replace all the rubber feet.
16. Reclip the x-fader module cable back into your apc40 and screw the module back into place.
17. Flip your new APC60 over carefully and replace the x-fader knob on the apc40.
18. Rock the discotheque, as my son Zack demonstrates.
Good luck and enjoy!
(that's another thread!)
Monster set up if you had the two 40s with the 20 in the middle...that would be awesome. post up when/if you do the mod.