Impossible Gliss?

So, I play baritone/euphonium but in our piece "Swing, Swing, Swing" arranged by Jay Bocook and in the baritone part near the end there is back-to-back glisses from Bb to G. My 1 valve slide works like a trumpet's I think so I've just been making Bb go in tune, out of tune, back in tune. Could anyone tell me how to gliss like that?