Bad idea. Without even going into the whole female issue, you now have ONE puppy that is not yet trained. The last thing you need is to bring another untrained (that's basically what "we don't have time for her" means) puppy (yes, at 1 she's really still a puppy) into your home to serve as a distraction for your current pup. NO training would get done. In theory it sounds like a good idea -- they'd keep each other company and serve as 24-hr playmates -- but the reality is that you'd have two untrained dogs that feed off of each other's bad behavior. If you think training ONE puppy is hard... try training TWO :eek:
Now for the female issue
It's not advised to have 2 female boxers in the same household because boxer females are generally very territorial, and bitch fights are serious business. When two females fight, they are not doing it to establish rank like two males would -- on the contrary, females fight to take the other one out. It's not a "one time and we're done" type deal, it will happen again and again as long as they are together. Of course not all females will fight, some females get along just fine all their lives -- really it just depends on the individual dog's temperament. But, it's more likely to happen than not happen, and it's too much of a risk to take.
The other thing besides gender that increases the likelihood of two dogs fighting is closeness in age. Less than a year is considered VERY close in age.
One last thing to consider: if the two dogs in question were to fight, they wouldn't start doing it until BOTH have reached adulthood. So basically, you'd have two dogs that got along great together until the
youngest one gets to be 2 years old... and THEN the fights would start, if they were going to start at all. And THEN you would be faced with the decision to rehome one of them, after years of growing attached to them both.
I know you've been having some "puppy regret" with the first one... I wouldn't want you to end up throwing TWO dogs out of a window 