When in high school, I played trumpet, french horn, flugelhorn, and euphonium in jazz and symphonic settings. I believe that this only helped my trumpet playing. As melodictrumpet said, french horn is harder to play than trumpet. You can pretty much play any note with any fingering, the slotting is all over the place....etc....it just made the trumpet seem more and more easy to play. And again, as melodictrumpet said, I believe that playing horn and euphonium shaped my trumpet playing. I became used to deep cup mouthpieces and larger rimmed mouthpieces (before then I only played on 3C's, now I play on 2 and 1 rims with much deeper cups).....that's a different story.....
So in my opinion, I think you should keep on playing both. Just find good ways to acclimate from instrument to instrument and mouthpiece to mouthpiece and you'll do alright! Hopefully you'll find that doubling helps your playing!