The simplest way to do it is a DPDT (double pole, double throw) switch. It has 6 lugs:
1 | 4
2 | 5
3 | 6
2 and 5 are connected to the toggle lever, so when lug 1 connects to lug 2, lug 4 connects to 5 - this leaves lugs 3 and 6 unconnected.
Same thing in reverse when you flick the switch - pin 2 connects to pin 3, and 5 connects to 6.
Snip the wire from the crossover to the speaker in two - you now have 4 loose ends...
Connect the 'Live' wire from the crossover to 2, and the 'Ground' to pin 5, then connect the 'Live' speaker cable to 3, and the speaker 'Ground' to 6.