My first reaction when reading the OP’s question was that they want to have a way to create “n” selections based upon what is selected in a single selection. Then, typing will overwrite the contents of all “n” selections, in reality doing a visual, character-at-a-time replace.
This posting (and corresponding Pythonscript by @Claudia-Frank ) does something similar and my be worth taking a look at: https://notepad-plus-plus.org/community/topic/11360/multi-selection-and-multi-edit