You will need to google it unless someone here has the numbers but if he has 2 cb antennas, that means they are phased.
It will affect your receive and transmit BUT they need to be spaced apart at the right distance otherwise they will do more harm than good.

If memory serves me right (and it usually doesnt) the gap between phased CB antennas should be around 9 feet.
One of the other radio guys may have the formula handy and can figure it out for 27mHz.

If it is around 9 feet then obviously you would be talking about mounting one at the front and one near the back of the truck.. kinda lame looking.
Personally I would stick with one and just buy a higher quality antenna. Something with a little higher gain than a cheapy whip.
Hope that helps!