As many people here will tell you, the two females combination is the least desirable because there's no telling how they'll potentially get along later on, when both bitches are mature (more or less when the youngest reaches 2 years old). It means that your girls can get along wonderfully for 2 years, be the best buddies ever, to suddenly turn against each other in deadly fights. Yes, deadly, because bitches will fight to the death (or until they are removed from each other's sight permanently). You can see how this can become a terrible problem to manage. Males usually fight to establish dominance, but will quiet down once it is done.
On the other hand, all bitches are different personality-wise, and many two boxer girls households are peaceful, too - a lot of people here have had very good experiences. In the end, it's a "bet" to take, hoping that the two females will get along for all their lives. The best combination is definitely male-female, followed by male-male.
I'm sure other people will give you tons of info regarding this

But a female-female combination can be risky.
Your girl is really pretty, btw
