How can I use the Built-in display as the primary monitor, and a projector (via HDMI) as secondary?

I have a Mac mini which has no display which I connect to it by sharing the screen to my laptop. The Mac mini runs apps which controls my live show's backing tracks, lights and video. The video projector is connected to the mini via HDMI. I would like the mini to still use the Built-in display as the primary, and the projector act like a second display so it can be used for the videos, but allow me to still see the Mini desktop and other apps via the screen share. However, when the projector is connected, the Mac automatically selects it as the primary display, and Built-in Display is gone. Is there a way around this?