How to do UI button Trigger only local player with a collider or mirror and multi client testing problem

Hello ,
Based on SDK3 exemples I try to toggle mirror and collider on my first world :

But I have a problem , it does not toggle for the local player only but for all players in the room
I’ve been trying to find for a while but I can’t find the problem
Are there any special options to check?

And I have an other problem,
for testing I try to launch 2 clients on “Local testing>Build and Test”
I can’t seem to be in the same instance to test my problem
someone managed to do it ?
Thanks in advance for your help
Best regards , Stephane