The word average means the second thing: the average of two numbers $A$ and $B$ is $\frac{A+B}{2}$, so in your case the average balance would be $\$ 11,813$, yes. (Well, actually it is $\$ 11,812.50$.)
However, that answer only goes as far as the meaning of the term. A better question is: for what you are trying to do, is it more meaningful to take the sum $A+B$ or the average $\frac{A+B}{2}$? That depends on what you're trying to do -- you didn't provide any more information -- but at least for me, if I have two bank accounts, it is the total amount of money which is more important than the average amount of money. In fact, I could hardly think of a more useless statistic for my purposes than the average amount of money in all my accounts.
Added: I just noticed that the two numbers you provided are already averages. This makes your question even less possible to answer purely mathematically. Really you are telling us: "I have two numbers $A$ and $B$, and I need to combine them in some way into one number. Should I do $A+B$ or $\frac{A+B}{2}$?" Answer: from a mathematical perspective, we don't know: it depends entirely on what you are trying to do. It sounds like what you were told was not clear enough, and you should ask again.
It is possible that someone with more experience working in a finance field would be better able to figure out what you are trying to do from your description, so you might have better luck on a different site, but I suspect that even a finance expert would find your question impossible to definitively answer.