I'll let you know soon how a T10 with 2 bodies and a 17670 works, as I have a couple of T10 bodies in route for my spare T15 head and tailcap. I know that you can stack 2 T15 Q5 bodies for a 2AA light.
I'm sorry to be confusing on T20 Q5. It is designed to run on 6-8 volts, ie. 2 CR123's or 2 RCR123's. I figure if anything can be fouled up, I can do it, I don't like juggling 4 batteries in the dark trying to feed a dead light (I use AW's protected batteries which when depleted, shut down). I imagine that I could put a depleted battery in the tube with a full battery and this could lead to a minor or major catastrophe depending on what battery failure occurred. Yeah, I know, it's not supposed to happen with protected RCR's. I can keep track of a single battery much easier, if I accidentally put the depleted one back in the light, no harm done, take it out and put the second charged full length battery in. And I won't need to do that for about 2 hours. That's why I prefer to use a 17670.
The Olight T20's low battery strobe kicks in about 3-3.2 volts. You have to trick the light to get it to run on 3.7 volts (which it appears to do much better than the Fenix P3D). If you set the light on level 2, then take out the 2 RCR's, you can insert a 17670 and it will pick up on level 2 without a hitch. Sometimes this is not possible, and upon insertion of a 17670, the light will light on level 5 or 6. If you let it run for a few seconds to warm up, then switch levels a few times it will eventually settle down into level 2. Sometimes it wants to run on Level 1 and won't switch to level two until it starts the low voltage strobe, then you can switch the light to Level 2. Level 2 is 120 lumens and about right for the use I put the light to walking in the local nature park. The fact that the light may not want to run on 3.7 volts is not a fault of the light, it wasn't designed to, but you can use it outside of it's design parameters successfully. On a 3.7 volt battery my T20 Q5 will only run on level 1 and 2, the lower light levels are pretty much lost, if you hit them it's an accident.