Hello Betsy,
Thanks for your post here.
Regarding to your requirement, it is not feasible to use both external camera and primary camera at the same time. From your description, if you'd like to share something while have a video meeting with others, you can save the image as a file in your computer and use the screen sharing feature to share it with others. For more information about it, here is an article for your reference: Share content in a meeting in Teams.
If you still want to use two cameras at the same time, we recommend you kindly give your feedback and share your experience via our UserVoice Service. Every customer’s voice is important to us. The related engineers will design or upgrade features referring to customers’ feedback and your ideas will help us to make our product and features better for you. We sincerely appreciate your continued support for our products and services.
Regards,
Seven