I had the same problem and submitted a support ticket to TeamViewer. Apparently, you cannot send certain Mac to Mac commands via Teamviewer, the OS intercepts them. They said my request would be sent to their Product team and added to a backlog, where it may or may not be worked on. So, if you're interested in having this be a feature, let them know. Before Logmein killed their free service, I used Mac to Mac commands all the time and miss this capability.
Here's their actual response:
Thanks for your reply. System key commands like CMD+TAB cannot currently be sent from one Mac to another Mac over TeamViewer. This is because the Mac operating system catches these special key combinations before TeamViewer has a chance to detect them.
However, I have forwarded this to our Product Management as a feature request. We always welcome suggestions and requests, although we can't promise if or when this feature might be implemented. Nevertheless, your feedback is very important to us as we want to continue to develop TeamViewer based on our users' needs and demands.
If you have any further questions about TeamViewer, please feel free to contact us again.