Irish seafood chowder with soda bread, although it’s not really a sandwich.

French onion soup and a croque madame or croque monsieur. Not usually eaten together but both are amazing.

Not going to argue with all the vietnames posts. Banh Mi and Pho are excellent.

The American staple of cream of tomato with a grilled cheese works as a great pairing.

If dumplings count as a sandwich then Chinese noodle dumpling soups are also excellent.