While it can be done, we don’t recommend keeping red runners and dubia together. Red runners are voracious and cannibalistic and will outcompete the dubia and soon you’ll only have red runners!
To sort them, you need two empty bins and a maybe a 2" painbrush. Shake the egg flats into an empty bin. Then tilt the bin so that the roaches try to run out. The red runners are faster (as I’m sure you’ve noticed 
) and you can use the paintbrush to push them into another empty bin. Repeat this process as much as needed. Let me know if that makes sense. It’s actually kind of of…like a roach horse race. 
Also, the bins look cool!! Are they some sort of mirrored plastic?
the visual on this…now i want to have an arachnid fair and send them all around with snacks to-go