Hi,
That's quite possible, though I would have expected one to be adequate... Mind you, if one battery in a pair has failed then it's pretty likely that the other one is on its last legs, too. The working battery you have might have been up to the job when it was new but not now.
I believe the idea was that Toyota could use smaller, less expensive batteries for cold climates. A single good quality one will probably be enough for you but it might be less hassle, if more expensive, just to replace both with decent ones the same size (I think that's "NS70"). Probably easiest to take them along to a supplier and ask for replacements... They'll probably dispose of the old ones for you, too.
Cheers,
W.