On buffetts, no. Everywhere else, probably yes, but is at the discretion of your waiter/restaurant. Most will. I've only heard second-hand that some won't.
There have been a lot of reports of people sharing between folks on the plan.
You might run into problems if you were trying to "treat" an adult who did not pay for the plan. You are not talking doing that, so you should be okay most of the time.
An individual restaurant, server, or manager could always decide not to permit it, but so far it's been widely permitted.
There isn't any rule against it. Unfortunately, as things are now, you could get caught in an overzealous net of enforcement, trying to impose rules that you're actually not breaking, but rules that Disney has no better way of enforcing at this time. Just beware that it is a possibility, though perhaps pretty remote.