I agree with Shockwave and this is why I recommend trimming the co-phase harness to tune in order to get the best match possible. If you know the feed point impedance of both antennas in parallel and the true VF of the two sections of coax being used, then you can get really close using the math. But getting the best tune possible while tuning side-steps the need to know these factors for sure.
You might be surprised however, to find that the feed point impedance at the end of a well grounded 1/4 wave element is near 15-25 ohms or less at resonance. If you don't consider resonance in your thinking, then you're making a mistake. True resonance is important to consider unless you like to operate your antenna system with high levels of reactance---and like our friend 'Doc tells us, "REACTANCE IN YOU ANTENNA'S FEED POINT DOES NOT RADIATE."
So, if you are seeing a 1.1:1 SWR match with a 1/4 system, you are likely seeing reactance (loss) at work big time and then you should consider ('Doc's) advice about reactance. You would likely make a better signal if your system showed you some SWR like 1.3 - 1:7 at the feed point. This happens with parallel feed lines in a cophase harness because you are working with a transformer and it is part of the antenna. If you have it right though, it will show a little SWR when you get the reactance tuned out of such a system and in my mind that is a good thing.
If you tune the harness, as in this case, you will compromise adjust for VF using the tuning process. To some degree you will also be affecting the resonance a bit, because the feed lines will effectively add a little length to the antennas. With a 102" whip on most vehicles the better the ground the lower in frequency you will likely see and the better the ground with 102" elements the lower the reactance (X=0) should be in the CB band, and this all tends to show a little lower in frequency than a 102" element should show.
In my experience we find a resonant match closer to 1.3 - 1.5 SWR in a co-phase setup---makes for a better signal. This is because you will not typically be able to reach a purely resistive load of R=50, X=0 ohms without adding losses in adding some sort of matching device near the feed point of a 1/4 wave element. This is true because we are dealing with an element located near Earth ground that will naturally show approximately 38 ohms of feed point resistance at resonance.
IMO, the closer your match is to 1.1:1 the more loss you will have in such a system, because you will probably never attain a 50 ohm match at the junction of the two short coax feed lines from closely space parallel antennas.
I will also add that you will find a properly tuned and setup co-phase system will show a very narrow bandwidth (<0.6 mhz) and that is a sure sign of such working correctly. If you try to operate very far from resonance you will see a sharp rise in SWR and considerable attentuation of both TX/RX signals. This is also what happens if you're system is not setup correctly and instead showing a broad bandwidth of (>1.0 mhz) or more.
A co-phase setup is a tuned device and is going to be part of the radiating antenna and it will be shorter than you think.
