In general, we don't recommend multiple users edit the same site simultaneously, but it most situations it should not cause issues.
As long as users are not editing the same content, our system should allow both sets of edits to go through without overwriting each other.
However, we do not currently have a "live editing" feature which would push edits on the CMS between different users in real time (however, we do plan to add this in the future).
If you know that you are going to be editing simultaneously with other users, your best bet is to frequently reload the page, which will pull in the most recent updates from any other users.