Co-phasing is applied when more than one aerial is used to transmit a signal from one single transmitter.
This isn't as simple slapping some wires & aerials together.. As stated in part of the aerials & signals section radio frequency signals are measured in wavelengths. A 27MHz signal is about 11.11 meters long (one complete sine wave). We have to ensure that co-axial cables & antennas are in tune with each other and then the radio as a whole. Aerials have impedance, co-axial cable has impedance, and the radio output also has impedance. Imedance is a component of Alternating Current signals, it's made up of a cables capacitance & inductance. These electronic terms are not imortant, but what is important however is that the impedance of all these cables and antennas connected together all have to balance out (so to speak).
You can't just connect 2 aerials all with 50 ohm co-ax. When 2 aerials are connected together (they are electronically in parallel) the 50ohms will drop to 25ohms, which is very undesirable for tuning, performance and radio life duration.
I wont go into detail in this section about the how to, but am just emphasising the why to. Co-phasing aerials produce different radiation patterns (also explained in the aerials & signals section) to standard ones, giving enhanced range in different ways.
That's all I'm going to say, please see http://members.tripod.com/~cb_antennas/. These pages are written and compiled by Scott - 2RP789, I had a look through and was impressed with his material. He calls it the ultimate guide to antennas for 11m, and I tend to agree!