It's always helpful if you have a signature that explains what your system is, and what macOS you're runing.
Just put your existing single M.2 NVMe 500GB into the OWC Accelsior 4M.2. If you have Mojave installed as your boot OS then go through the process of installing OpenCore and then installing Catalina. If you already have Catalina installed, then you may need to check that your OpenCore setup will continue to work, and no doubt you'll need to re-bless the EFI mount point. Maybe head over to the OpenCore thread to ask questions there about it.