Multiple turtles can become aggressive towards each (not sure if it's more common with one gender or another). Any time you have more than one turtle you will have to watch for the signs of aggression and if it gets bad (i.e. biting, cuts, fighting) you will have to be ready to give each turtle their own tank/habitat.
If you haven't already, read through the main site:
www.redearslider.com for all the info you will need to setup their tank or environment. The minimum size tank you'll need to start with is 10 gallons per inch of turtle. So if you are talking about already getting full grown turtles (boy's about 7.5-9 inches each; girl about 10-12 inches) That will be roughly be at least 180 gallons to house them together.
I suggest doing all the research you can prior to getting them to ensure you are fully prepared and setup for them. And on a side note: female RES have to have a nesting area that they can access as they need it.
Welcome to the forum. If you have more questions just ask.
