Rolled back from Catalina back to Mojave but I had two drives one for boot and other for saved files, realised old files are still around I rolled back from Catalina to Mojave because I was having issues, it was my first time to do so and foolishly didn't use Time Machine. 

I have two SSDs, one for boot and the other for my files, and I realise that my files are still on the other drive but I can no longer access them, the folders have the red stop sign.

Is there a way to access the files and use those folders again as my main user folder?
It's an easy task to make your old (Catalina) user folder accesible again. I don't recommend to use it as your (Mojave) user folder though.

Upgrading from Mojave to Catalina not only changes system files and apps but also may have changed some content of your user folder. After replacing iTunes and its library with the new triplicitas Music, TV and Podcast and their libraries, your old Mojave iTunes won't work anymore. Other files may have been "upgraded" too e.g. preferences files, Mail data folder etc.

You probably want to move your current user folder (Mojave) to Mac Storage again.

I assume that you didn't change your macOS shortname (=isa).


Rename the old isa (Catalina) folder:

sudo mv /Volumes/Mac\ Storage/isa /Volumes/Mac\ Storage/isa-catalina

Change file and folder permissions of isa-catalina to your current user and group id:

sudo chown -R `id -u`:`id -g` /Volumes/Mac\ Storage/isa-catalina

Move your current user folder /Users/isa to /Volumes/Mac\ Storage

You probably followed one of several how-tos (e.g. Move Your Mac's Home Folder to a New Location)
Move personal files and folders from isa-catalina to isa. As mentioned earlier I don't recommend to move content of isa-catalina/Library/... to isa/Library/... except prefs and config files of non-Apple apps.

To get your ("Catalina") music/podcast/photo/movie files back you have to dig into the respective folders and *.library files. Catalina's Photos Library.photolibrary is probably incompatible with Mojave's Photo.app.