What you need is a single pole, double throw (SPDT) switch with enough current-carrying ability to handle your system.
See the wiki link here: http://en.wikipedia.org/wiki/Switch#...t_arrangements
You would use the second one down in the matrix.
Your common wire would go to your controller. Connect your batteries in series, +ve of the 48V battery to -ve of the 24V. Connect the +ve wire from the 12V battery to terminal L1. Connect a wire from the junction of the series connected batteries (where the +ve of the 48V battery connects to the -ve of the 12V battery) to terminal L2.
When terminal L1 is active you have 60V on the common terminal, when L2 is active you have 48V.
ps. If you have trouble finding a SPDT switch with sufficient current ability, try finding a double pole, double throw with at least half the capacity you need, then wire the terminals together in parallel, this will effectively double the current carrying ability.