Yes, you can use two whips in parallel to get better performance at lower frequencies. In a mobile antenna, the whip is half of the antenna, and the car is the other half, especially if the whip is at the back of the car. The whip gives vertically polarized radiation, and the car give horizontally polarized radiation. For mostly horizontal radiation, the "whip" can even be under the car, invisible to the world. Also doesn't snag on the garage door. I put an aluminum plate under the spare tire well on my Nissan Sentra and loaded it with a tuner. Worked all over on 20m. All my engineer friends told me I was crazy until I built it and showed them. To see the invisible antenna, go to http://web.tampabay.rr.com/n4es
. The under car plate antenna really acts like the ground. The car is the antenna. Try your two whips. Short antennas are a capacitive coupling to the world. You are right. More is better. They will be too close to be directional on the low bands. You will excite the NVIS mode from radiation off the car, which is omni anyway, like a low dipole. Good luck and have fun.