you wait till they reach approx. 7- 8 months when they loose all their little black pin stripes on their head and reach the beginning of adult life in the meantime they should be kept in separate cages the sooner the better ...so id invest in another cage one for each...if they are opposite sexes this could cause some serious breeding issues down the road ... and welcome to the forum...

may i invite you to the forums tab at the top of this page then direct you to ...general parakeet chat and the medical and health threads and read all you can in both of those for EVERY question you could have and some you probably havent thought of as well...all questions you could ask are in there and all questions have been answered as well in those two threads...
enjoy your time to read while you let these birds become acclimated to their new surroundings and only do food/water/and cage cleaning for the next 10days...these are babies adjusting to a tremendous amount of changes in only just a few short weeks of their lives...its difficult and letting them adjust while you read is a good way to go...move very very slowly when you take care of the general maintainance so as not to further frighten them....and alway speak very softly to them as well.
