Disney currently allows you to do what you are suggesting. You'd have to pay for a second drink. Signature restaurants don't generally have huge portion sizes. You might want to pay for an extra entrée or an extra appetizer. I don't think one meal would be enough food for two guests.
As someone new to these boards, and working to understand exactly what the DDP covers, I think your original question makes total sense. I had also wondered about how sharing works/doesn't work at a 2TS restaurant.
What I am wondering is this: Is it possible for two people to share a meal at a signature restaurant by using just 2 TS credits? I would eat the appetizer, my fiance the entree, and we'd share dessert. This would give us the opportunity to try some upscale cuisine without spending too much.
This might be the same as your original question or might not -- I'm not clear whether you would have truly just 1 adult TS credit left on your last day (and how that is possible, unless one person is skipping meals or something) or if you would have 1 credit left per person in your party on your last day.
This is my first post ever, so please be gentle!
- Kristen