How best to split a stereo audio source (LR) to two stereo amps (LL+RR)?
For line-level audio (or approximately-line-level as in "headphone jacks") connections you can just split an output with wires and connect it to multiple inputs. To achieve any particular audio channel mapping you would like, you only need to make the corresponding connections between the TRS connectors' (T)ip and (R)ing contacts, and connect all the (S)leeves together.

For what I understand you want:







simulate this circuit &ndash; Schematic created using CircuitLab

(Using boxes for the connectors because CircuitLab doesn't have a more appropriate symbol.)

If instead of wiring you want to use commonly available audio adapters in a not too messy fashion, I would suggest the following parts:


1× TRS to pair of RCA (this can be found in both rigid adapter and Y-cable forms)
2× RCA to TRS (example, not a recommendation as I've had very mixed luck with this manufacturer's adapters but I don't have a better one)


Less esoteric than the second item would be to use two RCA Y-cables and two pair-of-RCA to TRS cables, but now you have a fairly long string.