[rabbit73]

If you have two identical antennas, aimed in the same direction, and are using a splitter reversed as a combiner, the coax lines must be the same length for maximum gain. You will be able to get up to 2.5 dB more, 3 dB because of doubling the signal minus the 0.5 dB internal loss of the combiner.

When the antennas are aimed in the same direction, the incoming wave front arrives at both antennas at the same time (when perpendicular), and the signals arrive at the combiner at the same time, so they add in phase.

This only works if the wave front is uniform across both antennas. If the wave front is not uniform across both antennas (like thru trees), you don't get the gain you expected. This explains why a 4-bay bowtie antenna sometimes works better than an 8-bay bowtie, like 4221 VS a 4228, because it has a smaller capture area.

If the two antennas are not aimed in the same direction, the incoming signals do not reach each antenna at the same time, so it is not necessary to have the coax lines the same length, because the same signals aren't going to arrive at the combiner at the same time anyway. Sometimes it works, sometimes not.

It is possible to adjust the lengths of the coax lines to different lengths so that one desired signal from both antennas arrives at the combiner in phase, but that often harms the other signals that might have been OK before adjusting the lengths.