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As I understand, the Cosmichorus V3 is stereo in and out via TRS jacks. If I run another pedals stereo outs into the Cosmichorus, and have only a mono out from the chorus to my amp, would this work okay?

I don't see why not. Where you're only running mono out of the cosmichorus I don't think you'd get the cool stereo effect unless you were running the cosmichorus with the stereo output.

I have an Echo 600 and want to have the delay trails when I switch the pedal off, but I don't want to run 2 amps.

If you run a stereo signal into the Cosmi, say a TRS output that is used with a TS (mono) plug, the Cosmi will disappear that stereo signal for you and leave you with just the mono one. In that situation the stereo channel, the right side, would be grounded via the input jack. That won't hurt anything, it's just the nature of mono/stereo routing, and the left channel will be unaffected by it.

But for your situation, you mean to say that the Echo 600 has a separate output for trails? And you're hoping to combine them into the Cosmi and keep the two separate? That won't happen, there's only left and right and never the two shall meet.. you'll lose either your main signal or your trails signal, depending on how you route it. I think. I'll ponder this more...