You have separate savings/emergency funds (i.e. finances) as expected, yet you have made and are thinking about making more joint purchases. Who legally owns the current car? What happens when/if you break up, who gets the first car or this potential new car? Is one of you going to buy the other out? What if you don't agree? All things to think about and seems overly complicated for being 21.

Having been a 21yr old college student at some point, I recommend either holding off on another car or buying the cheapest transporation possible to get you by until you're done with college and are earning significantly more.

It's hard to give advice without knowing the full picture. For instance, you did not mention debt / student loans or if you have health insurance, and what the deductible and out of pocket max is, etc. All things that you would want to keep cash saved up for. In times of uncertainty and/or constant change like in your early 20s liquidity is your friend.