is this only for visualisation or for audio play back/monitoring purposes also? By default, Reaper only plays back the first two channels of a multichannel item, however, it will always display all x channels in the item unless you change settings in the way you described. As far as I can see, there are no actions which do this with one mouse click. There might be scripts that accomplish this.
If it's for audio routing, you might wanna try my free "SonicAxiom_multi-channel patcher 8 ins to 2 outs" VST plugin (Windows-only) which lets you select two arbitrary, desired channels of a multichannel item to listen to. This doesn't affect item visualisation, though. If you need to be able to select channels above 8, you can use the appropriate 16-channel version. If that is not enough, I could code a 64-channel version.
By the way, you can always glue items while they play back the desired channels to get a newly rendered version, leaving the original version unaltered. However, it's not easy to change your channel selection once the item has been glued. This is why I coded the plugin
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Multichannel item. How to show just two channels on item by default?
Linear Mode
