I think it’s great that they’ll be pooling their money and sharing this way. It’ll be a great learning experience for them, and it’s a testament to their ability to find shared solutions.
I also commend you highly in your decision. As long as these two are living under your roof and/or support, you have a say. Stick to your guns. The Tracker is far less safe in the hands of an inexperienced driver than many other possible choices. I can see clearly where your kids got their abilities to think through problems and come up with answers.
When they’re finally on their own, then they can get whatever they want. Until then you may have to make some decisions that make you temporarily unpopular, but they know in their hearts that you’re making them because you care and in truth they’d much rather have it that way.