Two batteries.
 

I just picked up a used AX-10 and it has two batteries wired together and I was wondering if I really need them both at the sametime?

There both "Reedy lipo" 4800 pro mah. 20c::7.4v#708.

I know it won't stay charged as long but that's ok because I need the space.

No.

is it a saddle pack or two seperate batt's.......bob

....

Those would be some massive saddle packs!
It sounds like it's 2 lipos in parallel on the stock top tray.
You can separate them without any problem

The only reason I know of is some local scaler guys do "hands free" drives where the goal is to never touch the truck. No battery changes, no picking it up, etc. If you are stuck you must use a working winch or a tow strap from another rig to move you. if something breaks or you can't get back on your wheels you are out.
It's kind of a Camel Trophy endurance competition.

That's actually a pretty cool idea!

It's two separate batteries.