Before I upgraded my app to Catalina, I created a partition on my Mac laptop running Mojave and installed Catalina on it. I gave it the minimum amount of memory since I didn’t have a whole lot to give to it. I did NOT want to just switch over to Catalina since there are a lot of 32 bit apps I use on a daily basis like QuickTime 7 Pro, which I still find invaluable and haven’t seen a suitable replacement for. You would just need to select that partition as the startup drive and possibly share files between the 2 partitions.
Upgrading my app was not easy because it was video intensive and I needed to find new ways to do what used to be done natively in the RealStudio 2012 r1. I still use that for the Windows version as it’s still reliant on QuickTime.
Anyway, since then I got a new laptop that’s running Catalina and kept a 2013 laptop that I run Mojave and all my old 32 bit apps on. It’s a pain to have 2 laptops, but I didn’t have much choice. I frankly use my old laptop more than the new one.