If you want them to work simultaneously , I think it would be better if you have the clean signal from your guitar directly into both amp's inputs ... This can be done by a splitter , or if you'd like some cheap tricks and have some extra feets of guitar cables , you can split one cable that goes out of your guitar straight into the two amps at once.
For this , you'll need three cables , each with ONE jack at a ONE of it's terminals ... All you'll have do then is to join the other terminals together in Parallel (inside the cable , there should be an inside wire and an outside one , join each three similar wires from the different cables together) , and make sure that you insulate different wires inside the cables from each other well.
Frankly, I haven't done this , but I'm sure it should work well.