Perfectly reasonable. Mustachianism is about making thoughtful choices about your time and money, not necessarily about spending as little as possible, no matter how much you work.
My husband does something similar. He has a second job that he doesn't need to have, and he says the money is for fun purchases. But as he is pretty frugal, he never spends the whole extra amount that he makes. It just makes him feel better about the occasional splurge purchase.
How you and the SO split and deal with money is strictly between the two of you. If both of you are fine with how it's going, then no worries. Personally, I believe if someone chooses to work more, then they are entitled to that extra money. (As long as you're both in a decent financial position without the extra work and there's no other agreement in place.)
