@AdamDanischewski in this case you don't need an option, just put both items in the menu, and make both keybindable and bind one or both by default.
But note that I doubt that multipaste will help with pasting the output of `send selection to` since it would paste the multiple lines on every line I think (unless somebody checks Scintilla code or tries it and finds its different). There doesn't seem to be a way of telling Scintilla that the paste is a rectangular select, it says "Rectangular selections are handled as multiple selections although the original rectangular range is remembered so that subsequent operations may be handled differently for rectangular selections" and "pasting a rectangular selection places each piece in a vertical column." meaning the "rectangularness" is remembered internally to Scintilla somehow and is unlikely to be a property of the results of `send to selection`?