There are several ways to go about this but what you are essentially trying to do is a NBLRGB combo. Heres a high level overview on one way to do it without pixel math. There are some great scripts to help you accompish this with OSC. I would grab these if you dont already have them - Pixinsight toolbox from https://www.ideviceapps.de/PixInsight/Utilities/. Cosmic photons tools for image blend script https://www.cosmicphotons.com/pi-scripts/imageblend/ The continuum subtraction script from seti astro pixinsight scripts https://raw.githubusercontent.com/setiastro/pixinsight-updates/main/ and the pixinsight script DBXtract https://dbxtract.astrocitas.com/
If you have never added scripts, you should be able to find a quick youtube tutorial on how to manage your repository and add the URLs.
I would use your broadband image as your base image, do your normal linear things. Extract the RGB channels and set aside. Then stretch your broadband image - set aside.
Use the DBXtract script on your duo narrowband linear stack this will get you a fairly isolated Ha and OIII channel to process like a mono channel. Ideally you would then use the continuum subtraction script on your NB data (will utilize those RGB images you had set aside) - this will spit out non linear Ha and OIII data.
Then explore the Combine Ha with RGB or combine NB with RGB scripts in the toolbox to blend the data. Alternatively, you could use the imageblend script to blend the two images combined with masks - something along the lines of screening data with a color mask applied.
Use the selective color correction script and or curves to dial in your final color adjustments.
Now that you know the names of the scripts, you should be able to find the assortment of youtube tutorials to get you started.
Have fun!
