B&M Show Cards Rule

[fraserbrown]

If I show only one card to a player, does the show one, show all rule come into effect? I do this sometimes if Im playing with friends, I'll show them one card and then muck. Last night I did this and two players demanded to see both cards, I said since I only revealed one card I didnt have to, this resulted in the floor being called but was unable to make a ruling so I just mucked them in the middle of the muck pile. Was I right or did I have to show everyone both cards?