Nancy Marchand's nomination was actually for one show. The evening consisted of two one-act plays, "Black Comedy" and "White Liars," and she appeared in both. Although recognized for two different plays, this was one nomination.
I think Donald Moffat also only got one nomination, because at that time, the standard was 4 nominations for each acting category, and there were 3 other people nominated in that category besides him. If they counted the two roles as two separate nominations, they would have had only two other people nominated. The New York Times article from that year that lists the nominees also makes it sound like it was one nomination. Nowadays, the rules are different and he couldn't have been recognized for two different plays in the same category.
Nevertheless, it is an achievement for both actors to be recognized for their work in two different plays in the same season.
"What was the name of that cheese that I like?"
"you can't run away forever...but there's nothing wrong with getting a good head start"
"well I hope and I pray, that maybe someday, you'll walk in the room with my heart"