Hi Trumpetnick, I do this every day. I play in four organized groups, two on trumpet and two on horn. 1st horn in a community orchestra, 3rd horn in a community band, lead trumpet in a big band, and salsa-style trumpet at our Hispanic Baptist church. Each group has at least one rehearsal per week, and usually among them all there are two or so performances per week. The biggest issue for me is "setting" my ear for each instrument. when you see a written note, you want to hear it internally before you play it. That was my biggest hurdle to overcome. Initially, I would devote alternating days to each instrument, but I found that I have to play each instrument every day to keep my visual-to-aural linkage correct. I take horn lessons weekly with a local pro, and take monthly lessons from a commercial trumpet player. The horn lessons are invaluable! Don't be fooled - as a trumpet player you will be able to pick up a horn and play higher and louder than many horn players, but it will likely NOT be a characteristic horn sound. My self-taught horn playing sounded much more like an alto horn or a baritone horn in the high register than a horn (cor). Fingering was never a problem, left-handed for the horn and right-handed for the trumpet. The embouchure was tricky at first, until I internalized the feel of the correct horn mouthpiece placement. My teacher was indispensable for giving me feedback on connecting the proper mp placement and embouchure to the proper horn sound/timbre. I used a thick-rimmed horn mouthpiece to start (Denis Wick #5) to keep the same feel as the trumpet mouthpiece. I'm unsure if that was a good idea. It allowed me to play the horn immediately, but I had to give it up to "divorce" my horn embouchure from the trumpet set. I now use a Lawson horn mp that is much more convential and forces a proper horn embouchure. The two instruments can be complimentary, you absolutely cannot force or muscle a horn like a trumpet will allow you to (even though you shouldn't) and the trumpet literature will keep the high register from being scary on the horn, but it is a daunting schedule to keep facility in two demanding mistresses vice one.