Dedicated bracket
Is there a specific bracket for sistering together 2x4's, or is screwing them together sufficient?

What will double 2x4 be used for?

For sistering lumber used construction adhesive and screws. Start about 3'' from end and screw every 16'' to 24''. Use 2 1/2'' screws and drill pilot hole size of shaft of screw. Both sides for max strength.

Chuck

Thank you Chuck! The sistering is mainly to provide a wider surface for several sandwiches of lattice/hardware cloth/window screen which rest atop the 2x4's. They are the rafters of a pergola, and in eleven places the single 2x4 edge is not wide enough to catch the edge of the sandwich.
Lucy