Hi Martin, if instrument changes are allowed in the layout, Dorico will do instrument changes provided the notes (or other notations) don’t overlap.
You have a couple of options:
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add a second percussion player, who also holds chimes, and give the notes the 2nd percussionist should play to them. If you don’t want Dorico to number the chimes, you can put the two percussionist players in separate player groups. You can assign both players to the same part layout, so that all their instruments appear in the same “percussion score”
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add a notation on the glockenspiel staff at the same position as the chimes note, to prevent an instrument change. A chord symbol region can do this, for example, without adding extra notations