You can't assign separate preamps to the different Output To Amps - you can control the output level of the preset by the Output To Amp outputs. Output To Amp 1 has an independent level, as does Output To Amp 2.

I understood the original post to be asking if they could run their clean presets to one amp and the gain presets to a different amp. The Output To Amp 1 and 2 allows THAT to be done. You cannot have two different amp models in one preset regardless of the output levels of the outputs.