That’s because constructive interference (when both waves are in phase) permits optimal transfer of information. Perhaps another analogy: the two waves are like two cars. Both waves have a different phase relation, thus both cars move with different speeds towards each other. This means that at one point, car 2 will pass car 1 with a certain speed (depends on the phase difference between both waves). If you want information exchange between both waves/cars, it’s going to be extremely difficult, because in no time car 2 is past car 1. Reasonable information exchange is only possible when both cars have the same speed, and thus remain driving next to each other. When this happens, both waves are in phase, and they’re coherent (remember the temporal coherence). They interfere with each other and information can be transferred to each other. In shared dream terms this means that as long as brain wavefunctions are in phase, they can transfer information (i.e. dreamscapes) with each other. If not, they just exist next to each other, with little or no transfer possibilities.
Thus, interference is necessary to provide the conditions under which both dreamscapes can melt together and shared dreaming becomes possible.
About your example: I see your point very clearly now (I hope ). And to me it seems like you’ve explained exactly the same theory as I did, only from a slightly different point of view. The fine-tuning of both brains (putting them on 97.5 for instance) can be compared with fine-tuning both brainwaves until they’re in phase. As long as one of the two is not at 97.5, they have an irregular phase relation and a more or less destructive interference. What you described is in fact the creation of a series of conditions which make it possible for constructive interference to occur.
On the other hand… if you mean that both dreamers need to adjust their brain to the frequency of a third source, the Dream Band (analogous to the people in their cars, listening to the radio, an external source with a specific frequency), how then would you depict this third player in reality? The general dream plane perhaps? But then again, even if everyone needs to adjust their brainwaves to the right frequency of the dream plane, it still requires interference. This adjusting is required because otherwise you can’t interfere with the dream plane and transfer information. So basically it all comes down to the same thing: brain wavefunctions have to be in phase (with each other or with an external source such as a dream plane) to have optimal interference, thus making it possible for dreamers to transfer their dreamscape and other information to other dreamers and their dreamworlds.