If Spektrum wasn't able to help... it's unlikely I can since I don't even own any of their products. But I can mention some stuff that may apply here.
First... I'd ask Spektrum again in hopes you get someone that actually knows what they are talking about. The previous one you were in contact with may simply have been new or other situation ( like laziness even ). That aside...
It was my understanding from what I've read that both the AR72000BX and AR73000BX are the same with the exception that the AR73000BX can provide more voltage/amperage to (HV) servos. If that is/was true ( for the date the info was posted ), another thing to consider is how updated each unit's firmware is. Newer firmware could have added additional features that the manuals ( they seldom update these ) didn't include since they didn't have the features at the time the manual was created. So check which version firmware each has and then check to see if newer ones are made for each model. Do not mix the firmwares, always use the firmware made for the exact model/version you have.
Both units are simply a Spektrum DSMX receiver that is combined with a BeastX flight controller into a single device. I'm uncertain which model BeastX that these use but if you can find out which is compatible, perhaps getting a manual for that/those would include info about the programming screens they should have. If you do find manuals for the compatible BeastX controllers, remember they will unlikely contain the receiver information also.
So to summarize ...
Check that each unit has the latest firmware installed and if not, update them. Also make sure you don't mix the firmware, get the one made for the model you have and do not apply it to the other unless they show the same for both.
Check if you can find out which BeastX controller is compatible and look at that/those manuals.
Ask Spektrum again in hopes that you get a more experienced tech support person helping you. Despite them also including tech from another company ( BeastX ) doesn't give them an excuse to not support a product they are selling IMO.
