The laser beams these missiles ride are coded so each missile should be able to determine which signal it should be following and which it should ignore.
From my understanding, the beam consists of a series of lasers each broadcasting a separate digital code. I think there's four separate lasers, arranged to create a four quadrant, "+" patterned, conical beam. The laser in the top left quadrant continuously broadcasts a code that when read by the missile, tells the missile to turn down and to the right. Because of the angle of the lasers, the missile's rear looking sensor will only receive the code of the quadrant it drifted into. All four quadrants point the missile towards the center of the "+". Because of the physics behind the system, for all intents and purposes, it's jam proof as the only way to disrupt the guidance is to position something between the missile and the emission source for the laser, which is usually the launcher.
Herein lies the possible problem with launching multiple missiles. The second missile in the salvo will physically block the emission source for the first missile. Eventually the first missile will drift out of the second missile's "shadow" and receive the code to correct its path. However, during that period in the shadow it would be unguided. The more missiles there are in the salvo, the less accurate the first missile will be. The obvious solution is to have two offset beams, or channels, so you can launch two missiles and not have them block each others beam. You can also lock onto separate targets if the beams can be independently aimed.
I don't know how much of a problem this actually is, but given that using separate beams to launch separate missiles is the common practice, I assume it's the more reliable method.