Dear Marc Newman,
Good day! Thanks for posting in Microsoft Community. We're happy to help.
From your description, it seems that you want to add multiple trend lines in Excel on Windows device.
If the understanding above is right, generally, we can add additional trendline by clicking again on the green + > Trendline and select the trendline as per our need.
The screenshot below is a chart with one trendline and the other with multiple trendlines.
Hope the above information can help. If our understanding of the scenario is not consistent with yours, feel free to point it out.
Thanks for your understanding and cooperation! Hope you keep safe and healthy always.
Sincerely,
Rhoda | Microsoft Community Moderator