How can I simultaneously fill out my own forms and simultaneously do one or more of the other seven allowable returns on my Standard Online edition of TurboTax for Mac?

Go to Tools menu, select Return Manager, then Create a New Return.
You can work with several returns in progress, switching back and forth using Return Manager.
Any changes you make are stored in the cloud.

Note that of these returns , the one you are currently working on is treated as the Active Return.