Ideally, a female would be better than a male. A male will work, but it will constantly by bugging the female to reproduce numerous times durigng the breeding season. If you have no wish to breed, then I would get another female.
Size is important, but I would go more on weight than length. The closer you can get the two geckos to eachothers weights/lengths, the better.
Introducing a female into a male's tank is usually the preferred method, but I have had success with the opposite approach.
*There will be a heiracrchy established whenver more than one leopard gecko is housed together. The dominant female (the matriarch) will usually be bigger than the other females and usually will occupy all the preferred hiding places, along with eating first..............this is normal though, so don't freak out if you think one of the geckos is getting bullied or something.
I would suggest introducing two females if they are both at least 35 grams.
A male and a female being introduced into the same tank should be different though........the female must be able to properly go through the labor of reproducing. Which means the female should ideally be about 48-55 grams.
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2.4.0 Leopard Geckos
0.1.0 California Kingsnake
0.1.0 Copperhead
1.1.0 Eastern Box Tutles