Are you using a console? One trick is to run the pixel mad into the console via dmx in, then you can use a master fader to control the overall intensity of just the LEDs you need that way. Usually I don't have enough parameters to do this which is why I need pixel mad in the first place but it all depends on the situation. Just patch the artnet into the console as a DMX in and then you can re-send the merged data back out.

Otherwise a DVI parrot or DVI detective will work to trick that 2nd output you need,

unfortunately you can't stack 2 mixes on top of each other, even though technically you could have one mix below another mix at zero, pixel mad doesn't really respect that.
you can have mix 1 under mix 2 and have nothing on mix 1 but mix 2 will be displaying content and so pixel mad will output what is on mix 2. kinda weird but it is the way it is.

I can't think of another way to do this unless you have an awesome projector and you could send a larger signal than you need to it, hide pixel mad in the corner in another mix and mask it out in the projector itself.