I'm commenting to my own message because I was puzzled by the FireStik
article when it stated the two antennas should be at least 1/4 wavelength
apart.  I was pretty sure co-phased antennas should be at least 1/2
wavelengths (18 feet for cb) or more.  I did some more research on
co-phasing and found these two articles on a CB how-to web page.  The two
articles below are more accurate and explain better how closely spaced
co-phased antennas can work in a mobile installation.  I have recently been
interested in co-phasing antennas because I want to co-phase a couple of FM
broadcast beams to pull in a hard to reach station.

The below articles are for CB design but the designs will apply to amateur
radio and of course to FM-broadcast too.

From the "The Ultimate Guide to 11 Meter CB Antennas" web page (ok I laugh
at this, but it is interesting reading and I think the info is accurate).
Mobile Antennas
 http://signalengineering.com/ultimate/mobile_antennas.html
Performance Tips - Co-Phasing
http://signalengineering.com/ultimate/co_phasing.html

73, Ken -N5SWR