Mixing a layer feed (which is likely 16% protein) 50:50 with cracked corn will yield a feed that is 12% crude protein and even more deficient in lysine and methionine than the original feed.
That mix will also yield a calcium content of about 2-2.5%. That will help the roosters but won't provide enough protein for your hens to be productive.
You obviously read the feeding instructions since you know roosters shouldn't eat layer. You should have seen that the instructions also had a statement to the effect, "this is a complete feed, provide as the sole ration, no supplements are needed".
That means cutting 50% with corn isn't advised by the mill - who has the feed and assay as well as the data from 100+ years of research what nutrients chickens need.
Corn is already likely the main ingredient. I realize some people will tell you to feed corn but that is already in the feed.
It doesn't save any money either. At least where I live. Corn and scratch here is only a dollar less (if that) per bag than feed. The loss in egg production and health negates any possible savings.
DobieLover offered the easiest approach.
A more complicated option is to have a hanging rooster feeder that is elevated where the hens can't reach and a hen feeder that the rooster can't get his head into.
