Hi Angel44,
To be more explicit, every copy/paste operation will operate on the same source/target channel. If you want to transfer data from the left to the right channel, it’s not going to involve copy/paste; just select an area, make sure both your channels are shown and highlighted in the Channels panel (left and right), then go to Process > Amplitude > Channel Remixer and define in each output tab how much of the source channels you want. In our case, let’s say we want to keep everything “as is” in the left channel, and in the right channel copy everything that is in the left channel : in the “Left” tab I would define Left 100%, Right 0%, and in the “Right” tab I would define Left 100%, Right 0%. After applying this, both the left and right channels (within my selection area) will contain what’s in the left channel.