I believe that some helicopters use 24V batteries the same size as 6T, or something similar. I remember someone talking about snagging one from their flight maintenance unit after it aged out or something like that. Perfectly good still, but in a multi million dollar helicopter you don't take chances like that.
The question is, why would you want to?
About the only thing that using a single 24V battery would change is completely screwing you over if something happened to it, as opposed to only halfway screwing you over if one of the 2 12V batteries decided to fail completely. You'd also lose any chance of using a 12V vehicle to jump start your rig in that situation.
Look at it this way, with the military's pretty much unlimited budget, if there was a 24V battery that was better suited to this job than a pair of 12V batteries, they'd have probably switched over to it long ago.