Opening AccountEdge on High Sierra External Drive

I've been hesitating upgrading the OS beyond High Sierra mainly so I can still use 32 bit apps like iMovie HD6 and my accounting software AccountEdge. Accountedge has notified all Mac users their Desktop version will not be 64 bit compliant, sad to say.

Rather than getting into the expense of cloud based accounting, my thoughts were to move High Sierra to an external drive which will free me up to upgrade the OS on my main machine.

My question is, if I move High Sierra to an external drive and upgrade to Catalina, will I need to boot into the external drive every time I want to to use my accounting software or can I simply mount the external on the Desktop and open AccountEdge from the mounted drive. Hope that makes sense.