@GeorgeB some arrangers of marches are very unimaginative and write nasty parts for 2nd and 3rd cornets/trumpets don't they? My suggestion is to get your double-tonguing up to speed. It's far easier to cruise through a long stretch of semi-quavers even if you can single tongue at that speed.
A few years back a teacher gave me an exercise to develop triple-tonguing that is equally applicable to building double-tonguing speed and technique: Simply pick a scale that is comfortably in your range - maybe F major and start - (F) tktk tktk taaaaa (G) tktk tktk taaaa and so on, up and down the scale. Do it slowly at first, concentrating on getting the articulation clean and even. Getting the articulation clean is more important that speed ( +1 for the suggestion above to practice just "K" articulation too).
I won't buy into the discussion about tuku vs taka except to say that it's a case of what ever works in you head - dugu dugu works for me ☺