I ran into this dilemma while making some Christmas cards. The gold ribbon I had was either too wide & bland or too narrow. So rather than throw them both out, I sewed them together to make a fabulous ribbon that is just what I needed. And it was totally easy.
At first I pinned the ribbons together to feed through the machine. Then the pins became a pain in the butt, so I decided that it would be easier to just hold it together for the initial stitches. After the ribbons are secured together, you just let the machine do the rest, making sure the smaller ribbon stays centered. And you can't even see the stitch because I used shiny gold thread.