Almost my entire time enlisted, I had a seond job. Usually for the same reasons you losted. Cars is expensive and I like them.
But, here are a few of the hidden expenses you don't see coming. You are going to spend more money eating. You will expend mor energy, and if the hours start to infere with your sleep patterns, your body responds with a desire for more calories. So you end up grabbing a burger from that drive through window or a taco from this one. Not so much as you blow all of you money, but enough that you notice. Plus more coffe/Red Bull/ whatever you use for caffiene.
Taxes. You might unintenionally move yourself into a higher bracket. That means you are paying a higher percentage of your main income in taxes, and that has to be supplemented with the additional income from your part time gig, which kinda defeats the purpose. You of course won't figure this out until March when you see what you owe, or how little your are getting back.
Again, I did this for the better part of 7 years, and I really got sick of it. I never had any time, which is almost as bad as not having enough money. I felt like I was always on the go. It grinds on you after a time. Grabted I was also usually taking classes as well.
Its worse if you actually have a work ethic, becuase you stand out and get scheduled more, and as was mentioned, if you push back, you might stop being scheduled. Lost a few jobs becuase they didn't understand the concept of part time.
You and the missus working 2 jobs will cut into your together time and that will have an affect on your relationship. So if you go that route, you need to make it a piont to go visit her when she is working, if only to stop by, make sure she has had a chance for dinner (it she doesn't wait tables at a resturaunt) or literally just spend her break together.
I am old now, and can't get behind the idea. But when I was younger and didn't have a 2nd job I felt like I was wasting time. My wife had three jobs when we met and had a real hard time wrapping her mind around not needing to work 80 hours a week.
It can be done, and done well. Just recognize the pitfalls and you can aviod most of them.
Good luck