It's a good thing to never leave your budgie alone but, allowing two parakeets to be together untrained... is ... well... unhealthy for the individual relationships that Poppy, Sky, and you are supposed to have. Allowing the both of them to be together first rather than be with you first has let them create a bond with each other. If you let this continue, Sky and Poppy will become better friends. The whole point of having a budgie alone with you for a couple months(meaning 5 tops) is so that you could build a strong relationship with it. Petting, touching, talking to it, and playing with it for a few minutes are all good things that can build a strong friendship.
But to answer your question...you should do neither. Do not let them be together within the first months it comes home. This can be bad for both parakeets health wise, because they come from different places and can carry different sicknesses or germs. This could cause some life threatening problems for your parakeet.
You also should not put your parakeets in the same cage as parakeets can be very territorial and will in some cases fight to the death to be the "clan leader" or to gain respect or some other thing from the other parakeet.
But, in my opinion, Sky never needed another friend but YOU. Even though you are working, she may look foreward to having you come home. But if she gets bored, while you're working... she should have enough toys that will keep her busy. If not, she could start plucking out her own feathers. THEN she will need someone else for company.
But what you need to do now is keep them in SEPERATE CAGES, AWAY FORM EACHOTHER.
And when they call to eachother, they are looking for eachother. To see if they still remain in existence. They are looking for eachother's company. But you need to keep them sepatated. In different rooms. ALL THE TIME. But for only AT LEAST 5 MONTHS. You need to keep to this for Sky as well as Poppy's own good. Tough Love is almost always the best medicine for a parakeet. You may feel bad for them but remember, THIS IS FOR THEIR OWN GOOD.