To address the original question or goal stated in the thread title, I think what you actually want to accomplish is to move the front of your soundstage more toward the hood. Depth refers to distance between the front of a virtual stage and the rear of the stage, such as when we'd have a vocalist at stage front and center with the drummer typically at the rear of the stage relative to all other members of the band.
There are many possible ways could improve the sound of your vehicle. Adding a second pair of tweeters is one way to do it, but doing it right takes more than just wiring the new tweeter pair in parallel with your current speakers.
