As in the picture,
I have two horns played by the same player, and I want the horn I circled in the picture numbered 2. With the settings Engraving Options -> Staff Labels -> Numbering currently provide, when I choose Number together, it is numberd 3; when I choose Number separately, it is not numbered at all. It seems there is no workaround with other numbers.
Furthermore, there may be many more situations with the same problem. If the instrument can be numbered as I wish, it will suit similar situations like this one.
Thank you all for considering! With you all good luck.