What do you feed your dual purpose birds?
If a broody hen raises them they are raised with the flock from Day 1. I feed all of them an 18% starter/grower for the first month with oyster shell on the side for the hens laying eggs. After a month I switch back to a 16% protein for all of them, still with oyster shell on the side. They also forage for a lot of their food, vegetation, any creepy crawlies they can catch, and they get kitchen wastes and stuff from the garden..
For my brooder raised chicks (boys and girls) they get 18% protein at first but by either 5 or 8 weeks (depending on how crowded it is) they are foraging with the flock so they eat what the flock eats.
What age do you process them?
I try to never butcher a cockerel before 16 weeks and try to finish all of them by 23 weeks. I butcher my excess pullets around 8 months, after I've seen what eggs they are laying and have decided which I am not keeping with the flock. I retire my older hens at 2-1/2 years and replace them with those selected pullets. As one of my goals is to play with genetics I tend to replace my 1-1/2 year old rooster with a new cockerel every season. All this suits my goals and how I want to cook them.
How do I get them fat fast? Can I separate them from my main flock and feed them a high protein gamebird feed, and for how long?
Personally I don't bother, mine are typically raised with the flock until butcher. Size isn't that important with me. There are only two of us so I can get two meals out of a pullet. When I select which cockerel to keep for breeding size is a criteria, I like that extra meat for my lunches after we get those two meals.
You can absolutely separate them and feed them special. Sounds like you have a good set-up for that. I can't help with details for that as I don't do it.
I don't need a 10 lb bird, but it'd be nice if they dressed out at 4 at least.
With dual purpose birds you may be feeding them a while. If you got them from a breeder that breeds them for size it may not be too bad. But, as mentioned above, hatchery birds or birds from most backyard flocks are not bred for size or rapid growth. Even with a high-powered feed it can take a while and they may never get that big. If size and cost efficient feed-to-meat ratio is your criteria stick to Cornish X or Rangers for meat.
To get much meat on them, dual purpose cockerels are going to be quite a bit older than CX or Rangers. And they will almost certainly hit puberty. Age and, even more, the hormones of puberty will add flavor and texture to them. Some of us like that extra flavor, some call it gamey and do not care for it. The change in texture affects how you cook it compared to what you can do with CX or Rangers. Unless you take that texture difference into account you can wind up with some really tough meat.
