Heat up a pan large enough to hold one of the chuck roasts over medium heat. When hot, add the roast and let it sear until one side becomes brown and easy to turn to the other side with kitchen tongs. If the meat is stuck to the pan, it needs a little more time.
Flip and brown the other side. When done, transfer to a cutting board and when cool enough to work with, cut the roast into pieces. I cut mine into 1/2 inch pieces but you can go larger. I wouldn't go too much larger than 1 - 1/2 inches unless you are going to serve one or two pieces per person.
While the meat is cooling down to handle, add the other piece of meat. I'm assuming you purchased two pieces and not one 4 - 1/2 pound roast. When browned, repeat the process of cutting the beef into pieces.
When all the meat is cut into pieces, add it to the crock pot.