what if I record my ride with edge and garmin watch ?

It will be the same data, not the sum of all data.  I do this for every ride because as you know we tend to lose rides here and there being Garmin owners.  I never start a ride without a back up.

I think it will be two rides with (almost) the same data, depending on the sensors connected to your edge and epix. I assume that your training load will get false values if you don't delete one of the rides. With the Edge 530 I had to wear my watch (fenix 7) at each ride (without recording) because otherwise the training load wasn't calculated. Physio TrueUp seemed not to work correctly with these two devices. But with the Edge 1040 I leave my watch at home. Had no problems with saving the rides on the Edge 1040 so far.

I have don this and see both rides in Connect.  One from the 1040 and one from the Fenix 7.  The total mileage for bilking that day includes both.  I choose one and uncheck biking if I want to keep the data to compare tracks or heart rate of the watch and my chest strap.  I only tried it a few times.

You will have two activities. Both devices will record their own GPS track and the sensors they are connected each.
More or less, you will have two recordings of the same parameters.

It is unnecessary. Everything is synced afterwards via connect to the other device that in the end things like training load will not be correct.

I also believe that is not necessary to record a back up. I ride with my 1040 since it was released, something around 10.000km per year and didn’t lose any activity while the device was at fault.
I lost one activity because I stopped it and fumbled around in the rain instead of using the front door of my house (which I stood right before). In the rain the activity was deleted instead of saved. I blame myself.

I wear my watch while riding because I just don’t take it off, but I record on the 1040. Physio TrueUp takes care of the rest.