[PNW Fireguy]

If your original panel is not the same don’t connect it to the new string in series. I would be tempted to just replace the original panel with a third new one and wire them all in series. No extra wire to pull, no trying to find the path. Sell the old panel on CL or use it as a ground deployed unit if you need. With regards to paralleling the charge controllers you can but I would only parallel like controllers or those that can communicate to one another.