Connect a 1/4-inch instrument cable to the output jack on your guitar.
Connect the other end of the cable to the input jack on the signal splitter. You can use a dedicated signal splitter or any stereo-output pedal that has a signal-split function.
Connect a second 1/4-inch instrument cable to "output 1" on the splitter. Connect the other end of the cable to the input jack on the first amplifier.
Connect a third 1/4-inch instrument cable to "output 2" on the splitter. Connect the other end of the cable to the input jack on the second amplifier.
Turn the master volume on both amplifiers to zero to prevent unexpected feedback or noise when you turn them on.
Turn on the splitter.
Turn on the first amplifier. Tweak the tone dials until you get an undistorted, or clean, tone that you like. Add a little reverb for ambiance and adjust the master volume dial to a suitable level.
Turn on the second amplifier. Tweak the tone dials to get a sound that you like. Adjust the gain on the amplifier to a level that creates distortion.