Lady Ash, I saw this over in the Overlooked Tootle Puffer Gear Thread. Rhelton was the poster....
For our application married batteries are not that important unless you are charging internally. I am sure I will get flamed for that but it is the truth. Mooch has mentioned this also. It sure cant hurt to use the same batteries in multiple battery device. But if you are charging externally, you use a 2 or 4 bay charger and they charge individually it puts you back at square 1. When you charge internally, in parallel or series, the battery's will marry together as they discharge and charge together while staying in the circuit over their lifetime. After a period time that is exactly unknown to myself, the capacity and maximum discharge of each cell will become identical. If you should swap one out with a new cell it could be overpowered and possibly caused to discharge bellow its safe discharge range. This could lead to a battery venting or burning it out completely and be useless.
If its possible, it is best to use identical cells of same brand, capacity and discharge rate. If you charg multiple cells externally and happen to use a different one in the mod, its not the end of the world nor dangerous in any way. Even if you never rotated them the batteries do not know the difference. Cause once you discharge them, then charge them by themselves in a external charger the entire cycle ends and starts a new.
If you charge multiple cells internally via usb with a standard charger or balance charger you MUST always keep those batteries together. Use them as a pair, keep them together, tell them they are married and cant fool around with anyone else
And if one goes bad toss them both out and replace them.
I hope that made sense.
edit:
Just also be clear that its not a good idea to use new cells with old ones in any fashion. The whole "marry" battery thing was started because its much easier to tell everyone that you have to keep your batteries together in pairs for dual cell parallel and series mods. If you dont you will blow your face off! There is much more too it than just "marry" and you will blow your face off. Not so much its not understandable, but the veterans found it an easy way to keep the new people safe, rather than confusing everyone and possibly putting them in harms way.
I was at lunch when I posted that. You need to do a video of those big fat fluffy snowflakes because that's as close to snow as I want to be. 
