A 1.5 year old and a 6-8 month old female are probably too far apart in size to be housed together and the larger one will dominate the smaller causing it not to get enough to eat and to suffer from general stress. Younger and smaller geckos should be within a few grams of eachother in order to be housed together. The smaller the geckos, the closer in size they should be. Once they reach adult size (35+ grams) you have a little more leeway--for instance, a 35 gram adult will usually be okay with a 45 gram adult or so.