[albee1952]

Hmmm, I'm guessing this additional screen is using an HDMI input? Hopefully a more Mac-savvy user can chime in, but meanwhile, describe all the connections in detail(it will help get better advice). If you are using this to mirror your main screen(if you are not, then feel free to ignore the rest), here is what worked well for me: Using HDMI to feed my main screen(which is always showing the EDIT window), I went from the graphics card HDMI out to an active HDMI splitter(less than $40). The splitter had 2 HDMI outs and 1 went directly to the main screen. The second output had a 50' cable that went and joined a 50' USB cable(both fed through some nice mesh cable wrap). I keep a rolling cart(On-Stage Stands MX400) with a permanent shelf attached. On this cart is a 27" LED monitor and a mini keyboard with trackpad(both are attached with HD Velcro). The keyboard has the old PS2 connections so I use a short adapter cable to fit the long USB cable. When I need it, I connect the USB and HDMI to the rolling cart, put it where I need it and run the session from anywhere in the studio. The only catch is; the portable screen and main screen both run the same resolution(1920x1080). When not needed, I roll up and hang the long cable(but it stays connected at the computer end).

If the above doesn't apply, I would think you could go into the sound setup section(with the second screen connected) and disable it as an audio device, which might prevent PT from switching to it.