How do you do this? are you using 4cm? if so what's the routing look like? If not still what'd the routing look like? Is it straight from the Pre Out to the ABY then to the 2 amps? then how do you manage the Post Effects? Sound like a simple answer but sounds also like you're not using the POST effects that way
Guitar-TU3-FX8 input-FX8 PRE mono out-MXR Micro amp-AB-JCM800/Twin. One cable going to my board, two cables leaving the board to feed the amps.
I don't have any effects assigned as post, as everything runs straight into the front end of both of my amps.