You can serve these with lime wedges, or make a Thai-style dipping sauce of nam pla or soy sauce, lime juice, a little sugar and a few chopped scallions. Or yield to heresy and spike a little mayonnaise with nam pla or soy sauce.
Purée shrimp in a food processor (smaller is better) until you have a smooth paste, or chop and mash by hand. Add fish sauce. In a bowl, mix the shrimp purée, crab meat, egg, scallion, cilantro, chili, ginger and salt and pepper; add just enough bread crumbs to stiffen mixture a bit. Refrigerate mixture until you are ready to cook; it will be easier to shape into cakes if you refrigerate it for 30 minutes or more.
Season flour with salt and pepper. Film bottom of a large skillet with oil and place over medium-high heat. Shape crab meat mixture into cakes 1 inch thick and as wide as you want. Dredge each in flour, and cook, adjusting heat as necessary and turning once (very gently), until golden brown on both sides, about 5 minutes a side. Serve with lime wedges.