Definately put one boy and one girl in each tank. While two girls will be just fine in a divided tank, two boys will just flare and stress each other out so since you have two of each, you can do boy/girl tanks.
The only tank dividers I've ever seen are the ones made by Penn Plax
http://www.petsmart.com/global/produ...der&N=2#detail. They're good because they go up high and if you get a glass canopy there should be no space to jump through. There may be other brands out there but I've never used them so I couldn't tell you if they were good. The Penn Plax divider has tiny holes in it so the water flows through freely. I wouldn't get anything that is solid with no holes. Put your heater right next to the divider and put an airstone under or beside it and it will distribute warm water on both sides. You shouldn't need more than one heater. Have thermometers on both sides to monitor temp.
If you have the standard rectangle shaped 10 gallon tank the smallest divider will fit. You'll need to trim the side a little as I found it about 1/2 inch too wide. If the divider fits snug you may not even need the metal holding clips. The clips will prevent you from fitting the top on properly, which is why the glass canopy is better. Just make sure the divider IS snug and can't be pushed over by a fish. They aren't that strong so they shouldn't be able to.*

I've heard that the dividers for the larger tanks don't fit (are too short) but the ten gallon ones do fit with a little trimming.