Hey MechDiver,
I have dived with two Al 40s deco bottles (70' and 20') and one Al 80 stage with no scooter. I had no problems with trim mainly due to have the canister light on my right. Not something I like to swim with but it was actually part of my Adv. Nitrox and Deco class (had an excellent instructor). Anyway, back to the bottles... we did this dive with three bottles on our left as well as our Argon bottle. The trick is place the O2 bottle on first, because it is the last one you will use. Then you attach the stage bottle (Al 80) which is the one you will breathe. Attach the 70' bottle to the hip D-ring with just the top snap (the one close to the valve). When you are swimming the 70' bottle has enough slack to trail behind you and out of the way.
After you have used almost all of the gas from the stage, simply unclip it from your D-rings. The stage is empty and very bouyant. With the stage reg still in your mouth let the stage float up in front of you. Now you can reach back and move the 70' bottle up and clip to your D-rings as normal. Now you can switch to your back gas, pull the stage bottle down, stow the reg, turn the valve off, and clip it to your hip D-ring using just the top clip (like you had the 70' bottle before).
Now your ready to use your 70' bottle for your deco. After you have finished the deco using the 70' bottle, just make the switch over to your O2 bottle the same way you switched from the stage.
The key to using stage and deco bottles is that you only breathe off of the bottle that is on top of the stack (I hope that makes sense). Once your done breathing it, get it out of the way, by moving it back behind you.
If you haven't noticed I am a STRONG DIR advocate as I have found nothing that doesn't work in the DIR system. I also couldn't care less if you dive DIR or not.
Have fun and don't try three bottles until you have the two down first. It is tough to swim with.
Dive safe