Maya Island Air shows a flight from BZE to San Pedro Sula. Just a puddle jumper from there to Roatan. I did the Roatan to Belize trip a few years ago but not the reverse. That was on Taca. There is a ferry that runs but I'm not sure what the location in Belize or times are.
Last time I did BZ and Roatan back to back, I went all the way back to Houston from Roatan, stayed overnight and then headed down to BZ. Really didn't cost all that much more even with the extra night in Houston. Found the whole thing quite relaxing really.
A fish and a bird can fall in love, but where will they build their nest?