Yes, SI allows multiple users to edit a project simultaneously. The default mechanism for working on projects in SI is to allow users to check in their projects when they close them and check out projects when they open them.
You can enable Multi-User Editing, which allows multiple users to edit a project simultaneously, under Project Rules in Control Panel.
With Multi-User Editing enabled, you will see some changes in how you will work on projects in SI.
Check out this article for more details.