I created two objects, a hollow tube and a frame that I need to join so that the tube opens into the frame. I joined the two objects where I want them and I'm trying to remove the faces of the frame to open it up to the inner walls of the tube. I can't figure out how to put edges along the path where the frame's faces meet the tube's faces.
Well, I'd bet there was an easier way, but here's how I ended up solving this. I created a new object from the vertices on the inside edge of the tube. Then used that with a Boolean operation to cut a hole in the walls of the frame (though I had to move the frame away from the tube to make this work, then moved it back into place after the cut).