Place pastrami, corned beef, and broth in a slow-cooker.* Heat, covered, on lowest setting until hot, stirring periodically, at least 1 hour and up to 3 hours.
Set out bread slices and cheese on a platter, and mustard and coleslaw in small bowls. Cover with plastic wrap until served (up to 1 hour).
When pastrami mixture is hot, invite guests to make their own sandwiches, or assemble for them: Spread 2 slices of bread with mustard. Pile a generous helping of the meats onto 1 slice, then top with a slice of cheese, about 1/3 cup coleslaw, and the second piece of bread. Cut sandwiches in half, they're large, so consider sharing.
*If you don't have a slow-cooker, put the pastrami and corned beef with a little broth in a microwave-safe bowl, cover, and warm in a microwave oven.
Note: Nutritional analysis is per half-sandwich.