Official company line is NO SHARING. If you call Disney, that's likely what they will tell you. However, people seem to have a certain amount of luck asking their servers if they can share. It really just depends on your server (and possibly the restaurant).
Remember that if you succeed in sharing, add extra to the tip to cover whoever is eating off of the one plate.
Keep in mind that you can easily share at counter service places, because they don't know who's eating the food after it leaves the counter.
Other reports state that portions have also come down in the past year or two as more and more people use the Dining Plans.... depending on where you are you might not realistically be able to split because there's less food (especially since you no longer get an appetizer included).