Your plan was mine, as I have people waiting to take gerbils from me, I was to keep one male and one female and let the rest go in pairs. However out of 2 male pups only one survived from the last litter. This 2nd litter has only had 2 again (I will be sexing them aged 10 days on Friday).

You may not get a litter with one male and one female.

Yes you can sex at 10 days fairly accurately but telling what is what at 6-7 weeks when they can leave Mum is another matter.

I really hope you can keep both gerbils and a companion for each. I hope the tub is big enough - it needs to be 20 gallon size for a Mum and litter.