you can't open immediately on double caps. you have to build one lab building, then research a fleet logistics unless you do something nutty like scrap all your scouts and play blind for a while. then you have to research a cap crews upgrade. it takes several minutes and a bit of waiting for resources to get that 2nd cap out quick enough.
i just tried this, by the way, rushing out a 2nd cap as soon as possible. got an Akkan as the first one and a Marza as the 2nd as soon as i could. it worked alright (was playing 1on1 vs. hard AI) but it wasn't noticeably better than a conventional opening. cleared militias a little bit faster, but not faster than i would have if i just spent 3k credits on cobalts (which would be 10 of them).
it also screwed up my econ a little bit because i normally don't research the fleet logistics upgrade for a few minutes because i won't need it. the 40ish logistics left over after first cap and scouts is enough for 8 Cobalts, which is more than enough for clearing planet militias. so i had to take a 9% income penalty earlier than i wanted to.
bottom line is i would not recommend rushing for the 2nd cap. buy it when its a strategically good time to buy it, don't get it out there ASAP just because you can.