Welcome!
You will need a footswitch in order to select just the A or just the B channel conveniently. With no footswitch plugged into the jack, both A and B channels are on. With the footswitch plugged in, you can select either A or B channel. You can still use the channels separately without the footswitch by turning the "other" channel's volume off.
Here's the owners manual and schematic for the 260 and its footswitch. It's a simple circuit with just a couple of leds, diodes and a resistor.
As far as the nature of the footswitch circuit, a simple on/off isn't likely to work although no harm trying. I think it is likely it will just toggle between the A channel with the footswitch clicked on and both channels with the footswitch clicked off. I think the amp will interpret "off" as no footswitch is plugged in.
Enjoy!