The normal mode of operation is independent attitude sources for each pilot. the only good reason to both be on the same source is if one is already identified as failed. in which case (1) it's made very clear that you are doing so (and cross-check with the standby would be even more important than normal) and (2) would you want the system nagging you the rest of the flight about a miscompare with an identified failed unit that you've already addressed.
There are good reasons to base the comparison monitoring on the displayed data.