Cut octopus tentacles away from the piece that holds them together; cut into smaller pieces. Cut head in half.
Combine octopus pieces and shrimp in a skillet over medium-high heat; cover and cook in their juices until octopus is almost tender, about 25 minutes. Transfer octopus and shrimp to a bowl, reserving juices in the skillet.
Stir onion, salt, bay leaves, and black pepper into the skillet; increase heat and cook until juices are reduced and onion takes on the color of the juices, 3 to 5 minutes. Stir in tomatoes and sriracha sauce; cook for 5 minutes.
Return octopus and shrimp to the skillet. Reduce heat to a simmer and cook until flavors combine and sauce thickens, about 30 minutes.