Plug your one of your guitar cables into one of your amp's input jacks. Plug the other end of this same cable into the "A" output of an A-B-Y switch pedal. This pedal allows the user to choose between two different amps or use them both at once.
Plug another guitar cable into your remaining amp's input jack. Plug the other end of this same cable into the "B" output of the A-B-Y switch pedal.
Plug your guitar into the properly labeled "Input" or "In/Out" jack on the A-B-Y switch pedal.
Turn on both of your Marshall amps.
Step on the button that says "A or B" or "A/B" to select one of the amps. Pressing down on it once will select the "A" output and pressing down on it again will select "B" output. Press down on the button that read "A and B" or "Y" to send a signal to both amps simultaneously. Play to make sure it works.