My cage is similar in size to the white one you initially posted & i open the door for my two budgies whenever i'm home, which sometimes means only 0-4 hours of being allowed outside until the weekend. However, they usually stay inside their cage even if their doors are open, only coming out to fly around for a few minutes or walk around on the roof of their cage (I've been told that they are very polite birds for not exploring everywhere & pooping on everything). & my previous budgies did not have their quality of life affected when staying inside their cage most of the time.
Which is to say that, in my personal experience, two hours is sufficient. Just make sure to have enough toys for the both of them to occupy themselves with.
Heed Mizzely's advice, though... female budgies are very territorial, please keep them in separate cages for a while as they get used to eachother. I don't have experience introducing budgies to eachother, but i have heard of other people's experiences with budgies (especially females) attacking the new budgie, so i would recommend you do more research on that.
Good luck!