I have a sync - position on sprite...But I was thinking set client input state "x" and "y" to mouse.x / mouse.y on double click and using create sprite at Multiplayer.peerstate("peerid","x") and Multiplayer.peerstate("peerid", "y") should do the trick...I didnt try sending the mouse.x, mouse.y in the message directly...Do you know what is wrong with my approach?
There's nothing wrong with your approach, but it's rather complicate things, I would say it's a not-so-good practice rather than being wrong for something that simple. It's unclear with your Multiplayer.peerstate("peerid","x"), it seems it doesn't associated with any object. Check this first.
Send message will provide pin point accuracy at the double click event on peer.
If you use sync, the position might be a bit shifted (might even be outside of the screen) depends on the current mouse.x mouse.y on host.
The difference between sync and send message in terms of usage is as follow:
Sync is the equivalent of every tick occurence. (actually not equivalent, but same principle)
Send message is the equivalent of trigger.
Double-click is a trigger, so it is more intuitive to use send message. If you worry the mouse.x and mouse.y is hackable by peer, you can always restrict the condition in host.
What game genre are you working anyway?