At risk of throwing a spanner in the syncro.
Both can be true. I have just looked at the schematics for the MD83 which used a late model DADC, from Honywell if I recall, that clearly show Baro input from Capt’s Altimeter to DADC #1 and Baro input from F/O Altimeter to DADC #2.
The A320 and A330 ADIRUs also have Baro input to the AD (Air Data) part. It is, after all, a simple software fiddle for the Altitude output to the ATC Transponder to be related to 1013 Hpa and for the output to the Altimeters to be related to Baro Set.
I recall that the previous posts about the Classic are also correct regarding it’s, more primitive, system.
The same goes for CADC equipped 727s and 737-200s but I am not climbing into the roof space to verify that !
No offence to the Classic lovers – I liked them too