I am involved with rehabbing a 1906 balloon framed house. The upstairs bedroom cielings are rc2x4 joists. I am affraid if someone were to store stuff up in the attic the weight will cause the new cieling to crack or worse. The 2x4"s run 12 feet. I was thinking of running 2, 2x12's, 10feet across the joists in the middle and attaching them with the metal brackets. I don't know all of the names for stuff, but this would give added strength. and make the joists even. Is this a good or bad idea.
What you are referring to is typically called a "strongback" and is pretty much a standard part of framing in my region. It is usually built by nailing either 2 x6's or 2 x 8's into an L, and laid perpendicular down the center of the ceiling joists. It is more to help stiffen the joists to help prevent nail pops than adding center span strength. You are wanting to add more of a "beam over" , so be sure to get the ends supported on the wall plates and blocked laterally to prevent it from rolling. Hang both sides of each ceiling joist with a strap. I would use Simpson screws, available at the hanger displays, instead of nails, to prevent damaging the ceiling, and still do not encourage junk collection in the attic.