Consider a Turing machine with input alphabet $\{a,b\}$ that computes the following function:
$$ f(w, v) = \begin{cases} w & \text{if } \operatorname{length}(w) > \operatorname{length}(v), \\ v & \text{otherwise}. \end{cases} $$
I wish to use a TM with two tapes, the first of which contains the input string, say encoded as $*w*v*$. I'm not sure, having read several books, which tape is the output tape in this case. For example, the output tape for a TM of two tapes is the input tape or the second tape? If I use three tapes, is it correct that one of them contains the input, another is a working tape, and the third is the output tape?