doesnt matter how cose they are if you keep one alot higher then the other so for me my main drain was about 3-4" from the bottom of my over flow and my emergency was about an inch from the top or the "teeth"
imo a 1/2" drain wont keep up to much so you will have to make it bigger, and your emergency should never see water untill it needs to in an emergency.a small trickle is fine but i try for dry.
i also made my emergency using a durso and i found it kept up well if not better but i had all 1 1/2" plumbing and 1" for my return lines.
cheers
