You want to raise them separtely since the feeding requirements are vastly different between layers and meat birds. If your layers get that much protein, they'll grow too quickly and grow too large which causes problems with laying too soon and a generally bad FCR. In the same stroke, if you raise broilers on pullet developer, you're making it take a couple months longer than it should.
I got a batch of roosters from an egg farmer near me. I took 10 and put them on broiler ration (once we were sure which were roos). About 1 month later, I went back for 4 roos we missed the first time. They had been on a pullet developer for 30 days, while the original roos were on broiler feed. The size difference was simply staggering.