Carrot tail requires the gecko to have more of the orange in the tail, here's an example of a SHTCT:
The one could possibly be high yellow, but you can't always be certain until they are a little bit older.
Here's a careguide on leo's with sexing pictures, that way you can see the difference yourself:
http://www.thereptilefile.com/wiki/index.php?title=Leopard_Gecko
They seem a little too young to accurately sex, but you will know if it is a male due to the pores. If you do have one male, I recommend separating them until they are both around 1 year old and the female weighs at minimum 55g. The male will often try to break with the female, who in turn could breed early and die.
With using an under tank heater, make sure you are measuring the surface temperature either with a probe attached to the floor, or with a temperature gun (more accurate). A lot of heat mats get over 100F, and will require a thermostat or at least a rheostat to control. Every ZooMed UTH I've owned has gone over 100F (one was almost 120F) when not on a controller. Not something you want your geckos on!
Goodluck with them. Last May I had 3 leopard geckos and a crested, now I have 7 leos, 2 cresteds, a tokay, and a bearded dragon. Guess you can say it is addicting!