No, you cannot save the credit from an unused appetizer or dessert. However, you can choose to share a meal and consider ordering an extra entree--say for 6 of you, you order 4
DDP meals (4 appetizers, 4 entrees, 4 desserts.) Then 2 of you pay OOP for the 2 least expensive entrees. That way, you have 2 "leftover" TS credits that could be used for signature dining, a TS breakfast on the same day you do a TS dinner, or whatever. The 6 of you could share 4 appetizers and 4 desserts.
You could also choose to order 4 meals and divide them among the 6--no additional entrees. We have found that 2 TS meals can easily feed 3 people if they are in some agreement as to what to eat.
At some CS, they will stamp your receipt which will allow you to get your dessert later. They will tear the receipt when you pick up the dessert or scribble out the stamp to indicate that you have picked up the dessert. This is especially nice for ice cream desserts so that they don't melt while you eat. There is no requirement that you pick up the dessert right after you eat--just sometime that day.
You can also have your dessert boxed "to go" at a TS restaurant. Makes a great late night snack.