It's allowed as long as both people are actually on the dining plan, and it's not a buffet, or family-style meal. And of course you can always split a counter service meal (many are plenty big for 2 adults...like the ribs/chicken combo meal at Cosmic Ray's in Magic Kingdom...hubby & I often split that one.)
There are a number of dishes and restaurants that would not allow splitting a meal: or example, 2 people cannot split the all-you-care-to-eat skillet meal at Whispering Canyon. At Liberty Tree Tavern, 'Ohana, and Garden Grill, the food is served "family style" so everyone has to pay individually. Same at Buffets...everyone at the table has to pay. And Via Napoli (Epcot/Italy pizza place) will not allow dining plan patrons to split the individual pizza...you have to order a large pizza and that takes two meal credits.