My take on this question will be a bit different. Before I got my first chicks in Arkansas I chatted with my extension agent who put me in touch with a university professor specializing in chicken diseases and was part of the group that investigated any reported chicken diseases in backyard flocks or commercial operations. He told me that there had only been one reported case of Marek's in my county in the past two years. Also I planned to hatch my own chicks in the future which meant they would be exposed to anything the adults had. It was going to cost $10 to get my chicks vaccinated for Marek's. I used that money to get more chicks.
How likely is it that one of the flocks will give parasites or diseases to the other? Where are you getting the chicks? If you are getting them from a hatchery it is extremely unlikely they will bring in any diseases or parasites. If you are getting them from a feed store the risk goes up some but how much depends on whether they let customers handle the chicks or not. Most don't let customers handle them so the risk is pretty low. If you get them from another person I have no idea how good their biosecurity is. If you get them from an auction or chicken swap the risks really go up.
How tight is your biosecurity on your current flock? Are your chickens exposed to strange chickens? Do you take them to shows or bring in other chickens? Do you wear the same clothing, especially shoes, when you go to chicken shows, chicken swaps or auctions, or visit neighbor's flocks that you wear around your chickens?
I do not bother quarantining when I get hatching eggs and hatch them myself or get chicks from a hatchery or a feed store that does not allow customers to handle the chicks. That's the only way I bring in new chickens, that's pretty much a closed flock. I just don't give them that much exposure to other chickens and potential diseases or parasites. In other situations quarantine may be a tremendously great idea.
I do not believe in raising chicks in total isolation from the current flock. I encourage a strong immune system by exposing them to the environment they will live in from the start. If they are raised by a broody hen they get that. I have my brooder built in to the main coop so the chicks grow up with the flock, that makes integration go a lot easier too. They are exposed to a lot of whatever the adults have just by being in the coop, but to assure they get grit, get any probiotics the adults may have to share, and that they get exposed to anything the adults may have and their immune system working on any flock immunities they may need I take dirt from the main run and start feeding that to them about their second or third day in the brooder. I give them more dirt every three or four days to keep them working on flock immunities. I also keep the brooder really dry which cuts down on disease or parasite risks. By the time my chicks hit the ground they have flock immunities in place.
[GOD bless!] 
