All merging is bad, especially if either line is very busy. When trains start queueing they merge alternately. This sounds like a great idea, but because the trains are starting from stationary each time it's very slow. Better off giving priority to one line and allowing it to clear at full speed before the other line can merge. This is especially useful for joining a main line. Using pre-signals in a simplified version of the Right-of-way Lane Change and Right-of-way Depot you can force trains joining the mainline to wait for a gap. With a dual mainline this works even better.
All signals are combo signals except at points A and B which are entry signals, and the top right, which are exit signals. Adjust the length of the signal chain according to train length and speed.
Trains joining at point A must wait for the presignal chain along the inside lane to be clear. If a train arrives at point B before train A is out of the way then it will switch to the outer lane (at no speed penalty), but only if the presignal chain for the outside lane is clear. If it is not, the presignals at point B ensure that train B will take the line which clears first. The vectical section of track at point C is optional. It allows A to continue if the presignal chain for the outside lane is clear, even if the inside lane is not, as the train on the inside will move to the outside to let A out.