I think 4 watts in a regular CB is too wimpy for long distance comm. on
Interstates going at +65 mph. I've used twin phased 102" steel whip
antennas before with 1:2 SWR on my Chevy dually and still didn't have good
distance.

Co-phasing gives you a nice fore-and-aft radiation pattern, but only if the
antennas are far enough apart. 9' or 1/2 wave. Less or more than that, and
the pattern becomes a mess.

I frequently see "co-phased" setups where not only are the antennas WAY too
close together, they aren't even mounted with proper grounding.. Might as
well use a ball of bailing wire.