I don't think you have to but the pig will be happier. Plus you could sell the other pig when it is time to butcher. Just offer to take the pig to the processor and they pay you X amount per pound of hanging weight. I think I paid about $2.50 per pound hanging weight the last time I bought a pig from someone.
They gain weight faster when there is two, because they feel the need to compete for food, they have each other to snuggle with and keep warm in the cold months, and they have someone to be with. But that doesnt mean you need two, pigs can be just fine by themselves, and you spend less money feeding two