With Macro-Tech amplifiers the 1/4" inputs are in parallel with the inputs of the PIP-FX. You can use the XLR connector on the standard PIP-FX as an input, and the 1/4" of the same channel as an output to another amp channel.
The standard SST input module on the CE Series and Contractor Series (CH/CL Series) amplifiers have barrier block connectors in parallel with the Neutrik Combo connector. You can use the Combo connector as an input, and the Barrier Block connector of the same channel as an output to another amp channel.
The K Series amplifiers have a "Y input" switch that internally connects both channel inputs.
The CP660 has barrier block input connectors, so the input signals can easily be jumped from one channel to another.
With the D Series, you must use a "Y" cable.