XRNS is the format of the song. You can load it in another instance.
With two instances of Renoise, you can copy XRNZ phrases, one by one. But you must do it from the phrase editor, and reconfigure the number of lines. Too much work.
With two instances of Renoise, you can copy XRNI instruments, one by one, from the instrument box. This is not bad at all.
In both cases, make sure you have marked the corresponding panel to do the operation with the keyboard commands. Or, do it correctly from the drop-down menus with the mouse (right button …).
To test the tool, you should package it in an XRNX. But I should integrate the courotines so that it works in all cases. The above code is the main.lua of a typical XRNX tool. If you need XRNX packaging, tell me. For now, I can provide it without the coroutines.
It would be possible to do something similar with the phrases, to export the phrases from one instrument to another instrument inside the same song, through a tool window with selection. It would not even be necessary to use coroutines, because you are only copying text from an XML, with a total of 126 phrases at most, a very fast operation.
However, this operation is somewhat strange. It is possible to duplicate an instrument, where it contains all the necessary phrases and then modify the samples or whatever is necessary of the instrument (modulation, keyzones, etc).
All that involves copying text from XML files is very fast, although the iteration is with a very large range.