Each time I see a picture of Mike's dueling routers table, I intuitively love it... and I wonder what I'd do with a two router table setup. Of course this is coming from a guy who joined the forum less than two months ago for advice on his first
router purchase and I've thus far resisted the temptation to buy a second.
Back to topic, if I was attaching 1/4" masonite atop an aluminum frame for a router table, I'd want very
close support spacing and I'd want the router attached to the aluminum and not the masonite. If the aluminum supports were spaced 4" apart, I'd be comfortable. At 6", I think the masonite would deflect under pressure. My brain is much happier thinking of my tools as not moving at all, so I want movement below perception level.
I'd feel much better about the whole thing with the masonite attached to at least 1/2" plywood, and I'd screw the plywood to the frame from underneath. And if not for the support structure mentioned, I'd use a double layer of plywood as others above suggested.
You'll get more experienced advise from others though. I'm still a router newbie.