This would be from an EASA perspective only.
While I agree that only one pilot is actually tested at the time, said pilots ability to function as part of a crew is to be tested.
How about the followibg scenario:
During for example an Approach, you fail the Horizon or other applicable vital instrument of the PF only (pilot beeing tested), necessitating him/her to hand over controls to PM rather than keep flying on eg stby attitude only.
This alone would give an interesting insight to the Decision making in the crew.
Thereafter you "fail" the co-pilot once visual with runway (briefed to co-pic ahead of time, and with descent wx to give the poor guy a chance) requiring the pilot beeing tested to identify the incapacitation and proceed to land visually.
Probably several other scenarios that would accomplish the same as well, just my two cents/pennies/currencyofyourchoice