If they are young, even if they aren't cage mates, they don't need to be introduced again. Wow that's a big cage. the size of the cage should be big enough that if they were to fight, they can easily escape each other. if you need too, try to split the cage in two. rats have an incredible memory, and since it wasn't that long ago, they should be fine.

 

Thankyou very much for your advice, the boys are our first rats and I'm trying really hard to do everything right, my son will be so excited that we can go and pick their brother up tomorrow The cage is ridiculously big, I bought it second hand from a chinchilla sanctuary and the ratties look so tiny and a little lost in it right now!