Two comments,

Firstly the discrepancy in speed between the measured and the speed printed in the documentation is quite commonly observed, you may want to verify the barrel length.
Norma loads are measured with 24" or 61 cm, which is probably more than your Chapuis ( 60 cm )

The 156 grains oryx in 7mm from Norma is in my experience to hard and will not expand properly at the speeed obtainable in your double rifle. I suspect that it was created for the 7mm remmington magnum speed range. I have shoot a number of Boars and Red deer with the Norma Oryx and am not too happy with the result

156 gr norma oryx C.O.L.3,268”
powder min vel Max Vel
Norma 204 49.0 2428 51.5 2674
Norma MRP 52.0 2536 54.9 2720

The quest for finding a load that will shoot reasonable together AND have the speed required for proper bullet expansion is both time consuming and expensive.

The first question is if the first and second barrel keep their bullets reasonable together, this is best tested with two paper targets 50 cm apart. please remember to shoot standing so that the recoil and barrel movement simulates a hunting situation.

The time difference between the first and second shoot must be within 2-3 seconds. Also remember to let the barrels cool 15- 30 minutes between two consecutive shots from the same barrel.

You also may want to check the trigger pull weight, I assume that you shoot the first shot with the set trigger, and the second without , which in my experience is one of the root causes of bad grouping.

Keep posting the test results

Regards,