if you have 2 males, 1 15inch and 1 18inch then personally i would keep the 18inch because its easier to handle but i suggest if you get a female the same size and just leave them to it. dont encorege it by hubernating them. the chances of a big healthy litter with a female of 18inches are alot slimmer tham with a female of 25inches. it is better to leave them as long as poss before mating. i have 1.1 marcys in the same tank. i would perfere if they dont mate next year but i am not going to seperate them or encorage them atall. i am just leaving them to it.