Chicken diseases can be spread in different ways, through the air, through drinking or eating from the same source, eating each others poop. The more you can separate them during quarantine, the more effective quarantine will be. You can take it to any extreme you want to, changing shoes between tending to certain groups or using different containers to carry food or water.
The length of an effective quarantine is generally considered to be 30 days. Most things will show up then if they are going to show up.
What are you looking for? Parasites such as mites, lice, or worms. Some people automatically treat new arrivals for these without even looking for them. Sounds logical to me. Diseases can manifest themselves in many different ways. Strange poop. A chicken that fluffs up and is inactive. Difficulty breathing or gasping. One that does not eat or drink. Lots of different things.
This type of quarantime is only effective for diseases the chickens have recently come into contact with. If they come from a chicken swap or somewhere they have been in contact with strange chickens recently, quarantine can be quite effective. It is possible that a flock has developed an immunity to a disease and will never show that disease no matter how long you hold them in quarantine, but they can still infect and possibly kill the other flock once they are introduced. I'll use coccidiosis as an example of this, although there are others. To check for this, you can select one or two chickens from each flock and quantine them together in a third location.
Many people don't worry about quarantine and just put the flocks together. Whatever happens, happens. A lot of times this is not a big deal, but sometimes disaster strikes. I can't tell you how likely it is that any of the flocks are diseased or how much risk you are taking doing this. It depends on your risk tolerance and how inconvenient it is to do a proper quarantine. Some people quarantime every time. Some never do.
I can't tell you what to do. I take the risk of disease seriously enough that the only way I add new blood to my flock is to get hatching eggs and hatch them myself. No living chickens come close to my flock. But lots of people go to chicken swaps and bring home new chickens and never have a problem.
If the two people you are getting them from know what they are doing and would recognize a disease when they saw it, you will probably be OK. But I sure cannot guarantee anything.
If I read your post correctly, you do not have chickens yet. You do not have a flock at home. If I have that right, then I would not worry about it too much unless your are getting a huge amount of chickens. The more chickens you get the more money you have tied up in them, and a bigger risk.
Quarantine is really to protect the flock you have at home. You do not want to bring in a disease, and wipe out your whole flock. However, by mixing two different flocks, you do run the risk of one flock infecting the other flock, and winding up not having a wonderful chicken experience, but a bunch of sick birds.
imho - you need to be a little bit heartless, do not take or buy anything that looks the least bit raggy or sick. For the most part, although not always, sick birds look sick.
If I was talking about 5-6 head in one flock and 5-6 head in the other flock, and they looked pretty healthy to me, I inspected their housing, and it looked fairly decent with healthy animals, I would put them all together.
I think if you put them all together in a strange place to all of them, while you will have a few pecking order scuffles, it should not be too bad, and get over quickly.
ps. if one of two dies, I feel bad for a bit, but I do get over it. Some people have a hard time with this, and if you are like that, then you should be more careful.