10x20 coop = 200 square feet. Currently your hens are experiencing about 4 square feet per chicken indoors plus 12 apiece in the run. Doubling the number by adding your friend's would mean reducing that to only 2 square feet indoor and 6 square feet outdoor. Which is really fairly crowded.
It is one thing to do that with birds acquired all at once but IMHO there is a pretty high likelihood of having at least SOME pecking/cannibalism problems when you are mixing two strange flocks at that stocking density. Whether it would be modest or catastrophic nobody can say in advance. But it will not be peaceful, I will betcha anything.
Bear in mind that once cannibalism/pecking starts, it cannot always be cured simply by reducing density or removing "the offenders". Sometimes the only cure is to make a lot of soup stock and start all over again.
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I know the chickens pretty well, that isn't a concern for me. I deal with this stuff with cattle, quarantine. I read an article a few years ago that if you were bringing animals into your farm that unless you cleaned them head to toe, inside and out then a quarantine was almost worthless. I always keep stuff separated for a few days then check temp. etc. but, poop still happens.... to me anyway LOL
Well, it's rather different with chickens than with cattle since there are so many chicken diseases that cannot really be treated/cured and in some cases cannot be eradicated once introduced to the premises... and even just in terms of parasites, thinking about the potential to have to treat 100 chickens for mites or lice or leg mites makes my head hurt...
...but of course it's a personal choice.
Me, I think it has the potential to go pretty badly wrong in a whole variety of ways, and would not do it, but of course everyone has their own idea of what's a reasonable risk.
Good luck with your decision,
Pat