Currently, this is not possible. However, this is a very common enhancement request and it's on our Product Roadmap. Once we have more specifics about ETA, we’ll make sure you are the first to know.
For now I can suggest the following: assign one resource as a Project Manager and another as a Work Item Manager (at the project level). That way both users will have access to the project scheduling aspects including the settings and resource assignment.
Please find more information here: http://usermanual.clarizen.com/#1-4-1-about-user-types/roles$ProjManager.
All the best,