Hi Thibault,
Thanks for getting in touch. At this time, there is no built in way to sync a calendar across multiple sites, within or without an MU installation. However, you may be able to achieve something similar with a third party plugin such as FeedWordPress. I recommend that you download our free core plugin, The Events Calendar, and test to see if that will work for you. If it works nicely with FeedWordPress or a different solution, it’s very likely that our Events Calendar PRO add-on would also be compatible.
If you’d like to encourage us to add this as a built in feature, head on over to our Feature Request forum. Someone else requested this and you can add your votes. That way we can gauge customer demand and then prioritize our features accordingly.
Cheers,
Leah