Yes, it does take two people's credits to order one of these menu items. Just back today from WDW -- at Yak & Yeti, DD & her BFF had the Brownie Sundae for Two as their dessert. It counted as the dessert for each girl (adults) It was most definitely a two-person dessert-- huge.
At Mama Melrose, on the Fantasmic Dinner package, DH & I had the Calimari for Two appetizer (you get an appetizer with the Fantasmic Dinner Package on the regular/basic Dining Plan) Again, that counted as the app for both of us.
If you are dining alone on the Dining Plan you cannot order a "for Two" item...