Melt the 2 tablespoons butter and cheese in a double boiler over simmering water.
Blend in salt, Worcestershire sauce, ketchup, and cream.
Add crab meat and oysters, including any liquid.
Cook until edges of oysters begin to curl, about 10 minutes.
Drain mixture, reserving liquid.
Set shellfish aside.
In top of double boiler over direct heat, melt the 3 tablespoons butter and blend in flour, browning very lightly.
Gradually add liquid, stirring constantly.
Bring to boiling and cook a few minutes.
Stir in crab and oysters; heat over hot water.