Those who say that both cards must be shown are right imho although coprey is very much the voice of reason and I agree with him - we had a simlar (although very light-hearted) situation at Aspers last night where the player showing the card which paired was told to show both:
"Do I have to?"
"Well, only if you want the chips"
I think the key point is that we need a standardised set of rules covering all situations and available / displayed in all card rooms with Pub Poker leagues also signing up to these rules (if you play in a pool league, this is the situation - I accept poker is more complex).
Another example of this is "calling the clock". In the WSOP any player at the table can do it (reference Tiffany Michelle 2008 ME), In APAT only players in the hand can call it, but at the WCOAP DTD rules took precedent, i.e. any player at the table - I"m not saying that"s wrong, just pointing out the inconsitencies.