Many thanks to Brad at RSG, I was able to resolve the issue and it was indeed related to the CommandMapping.ini.
I had thought that setting the RXP 530 to COM2/NAV2 with "Master" RXP 750 on COM1/NAV1 was all that was needed, but I needed to manually edit the CommandMapping.ini file also (actually Brad made the changes and I just did a file compare to see what he changed) ... modified section headers:
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