Try this:
Practice slightly below tempo. Feel a pulse that is twice the normal beat (I don’t have the music in front of me and I don’t know what the time sig is off the top of my head… e.g. if the beat is quaters, count eigths) If you want, put a metronome on that. Now when playing, play each hand together (e.g if the trill is on C and B, play C and B together with the right hand, then CB with the left, etc.) Eventually move up to tempo. Once you have internatlized the number of trill sets (so that you’re not counting but feeling the number) you can start actually trilling.
I hope that works, but I can’t say that it has worked for me – I don’t have this problem so haven’t needed to think of an exercise.
~Sam
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