Greyhounds can become VERY competitive when running together, and things can get out of hand. It may take up to a few weeks or longer before you feel comfortable removing the muzzles outdoors. Close supervision is also a must while outdoors, both while muzzled and once the muzzles are removed. It is not always easy to know when you can consider both dogs as trustworthy. You will have to be the judge of that, based upon their behavior during the entire period.
Many people feel that it is best to mix a male and a female to avoid same sex conflicts.
In fact, some of the best experiences we have had were with females. If you want two of the same sex it can work, but you just need to monitor the situation a little more closely. You may not automatically be seen as pack leader anymore. You may have to work to maintain control. There will also be more jockeying for position between the three dogs.