I have three females with no issues. I do need to qualify that statement by saying that one is a 12 month old puppy and it is still yet to be seen of whether or not there will be issues when she hits the age of two and starts finding her pack ranking as an adult bitch. (I've also put *years* into their formal training and prepping them for showing in the Obedience and Conformation rings. This means that my girls are much more "tuned into me" than many other's dogs are to them. If a quarrell erupts, it's always been between a bitch that doesn't live in my home and one of my bitches, and all it took to stop it was me yelling "HEY!!" at the top of my voice.)
Two of my bitches are spayed and the only one that has been left intact is my puppy bitch (because I'm showing her). There is some thought that fights are much more likely because of quarrells over breeding rights, but I think it's been largely unproven. Many breeders have multiple intact bitches in their homes with no problems at all.
Also, as Willow said, opposite sex pairings are optimal, but much more important is the individual personality of the dog (ie: two alpha wannabe bitches = recipe for disaster). You are much more likely to have a dog and bitch get along than two bitches or two dogs.
Having said that, if you're not planning on showing them, health testing them and breeding them, you should probably consider spaying them, simply because it's healthier for them in the long run. As entire bitches age, they run the risk of contracting deadly pyometra and incidences of mammary cancer is much, much higher with each season that the bitch is allowed to have. (Not to mention, no worries of messy, stinky seasons, and having to guard your bitch to ensure that unwanted breedings do not occur.) Pyometra is especially alarming to me, simply because it can manifest itself in the form of "closed pyometra" which shows no visible signs, usually none before it's too late for the bitch.
The reasons above are why my two eldest bitches (6 years old and 4 years old) are altered.
As for introducing them, this is best done on what is called "neutral territory", or a territory on which neither dog considers to be "theirs". This can be done at the park, or in a new neighborhood, etc. It should also be done on leashes (that is, as long as neither is leash aggressive), and their body language should be closely monitored. (ie: Look for signs of posturing, stare downs, dominance, etc and be ready to act if it turns ugly.)
Good luck - I hope this helps!
