I have a score where he notes the orchestral clash cymbals like this :
and I thought I’ll keep that. is there a simple way to do double stems ?
or do I just add a “xylophone” and use that staff to note this, with two layers (or something)..
You could make a “clash cymbal kit” containing two clash cymbal instruments. You’d just place them on the same line but one up-stem and the other down-stem.
There may be drawbacks from using a kit that don’t immediately come to mind.
If you’re using a kit, you can also enter a note on the same position in a different instrument, move it to Cymbal using Alt/Opt+arrow, and then change its voice with Edit>Notations>Percussion>Change Voice>Extra Down-stem voice (or from the context menu like I did - make sure to leave your selection intact after moving the note).
(You might need Extra Up-stem voice for that last step, depending on how you defined your kit).