Conventional wisdom says that dogs will be more likely to get along if they are opposite genders and/or different ages or sizes.
Until recently, I had two females. One was a couple years old when we got the other as a pup and they were generally great pals. At maturity, they were close to the same size and had a couple ugly incidents but they weren't the fight-to-the-death battles you sometimes see between two bitches.
It's a crap-shoot no matter what you do. I would never get a second dog - regardless of the gender, age or size - without a meet-and-greet with the first one. If they hate each other on first sight, it's probably unlikely they'll change their minds later on.
People make it work all the time. I agree with what RonE said about conventional wisdom and opposite sexes. But it's more about each dog's personality and tolerances than about gender. It's also about how much you are willing to adjust your lifestyle around each dog. For example, some families only want dogs who will play with their resident dog. Some households just need for each dog to get along. Some people (like me) are fine having dogs who don't get along and use crates and pens to keep dogs apart when needed. I know that's not specifically what you asked about so the short answer is.. depends on the dogs.
I have had 2 females since they were 8 weeks old, only had the occasional fight, but I blame it more on their personalities than genders. I also have several coworkers that all have dogs of the same gender and they do fine together. I have heard of problems between unspayed females, and my male Sam hates intact dogs of either gender for some reason, but I always alter my pets so I can't really give you stories for intact dogs of the same gender living together.
We've never had any issues with our same sex pairs beyond a couple 'disagreements', but it's good to be aware of potential problems. Sometimes even with two that have grown up together, when they hit maturity there can be scuffles.
Certain breeds are also more prone to same-sex aggression. (I would never get another female Great Pyrenees while we have our girl Maya, but she is fine with our female mutt who is a tad smaller and younger than her).
I have had many same sex pairing without any issue at all. That said, I am a fairly rules oriented owner and very active with my dogs. It can work but as others have said...when you get two that do not get along, it is hell. It is generally worse with females vs. males.
Right now I have four females and two males. One male is neutered the other one isn't and all but my youngest female, are spayed. I have never had a problem with any of them but it does depend a lot on their temperament and they are all good. They are all together during the day when I am here but I do crate the three bigger dogs if I am not home. That is mainly because I don't like all six of them loose when I am not here.
I had two Unspayed females breifly (one was a foster) and they loved each other. I currently have a 6 yo spayed pom and a 1 yo spayed lab and they are not buddies but they tolerate each other just fine. In fact, the lab treats the pom with gentleness and respect, even when the pom growls at her to get out of her bubble. Previously, I had a male pom and a female pom (both altered) and they were best buddies. So I agree with what has been said that there is no hard and fast rule except that the dogs should meet before an adoption goes through and you need to gage the temperament of the dog already in the household. Some dogs will not like anybody, male or female, and some, just the opposite.
We had 2 neutered males for a long time without any problems, but as others have said, it really depends on the individual dogs. We took Toby with us when we went to look at Cameron. If there had been problems we wouldn't have gotten Cameron.
Husband and I both have had nothing but girls (2 at a time) growing up and now as well. I even had 2 intact females.
No issues outside of the occasional snark. It really is dog dependent. Some breeds/lines of dogs are for sure prone to same sex aggression.. but for the most part I don't really believe that same sexes are that often an issue. Opposite genders also fight.
I have four wolfdogs, 3 intact males and one intact female, except when the female is in heat, they all live together with just the occasional snark, no major fights in 8 years. They eat together, play together, walk together, go on trips with me together, just my pack and, they all get along.
I have two females. The older one is 10 years old & the puppy is 9 weeks. They get along just fine. Sometimes the puppy wants to play too much & my older dog will let her know when she doesn’t wanna play. No issues so far.