As it has been described on this forum in various threads, two reeds sounding together will sometimes "suck" the pitch together, making them sound unison even though they are tuned a few cents apart. This is the reason that it's VERY hard to tune a tremolo with less than about 5 cents - the reeds will suck the pitch to unison. On the other hand, this makes it a little easier to tune bone dry, as you will clearly hear when the reeds close in together. I find that this varies with pressure - if the reeds are not of the best quality, or has been retuned a lot, they will suck together when played loud. I was tuning some reeds a couple of months back that had been tuned up about 40 cents (from the original low pitch - 20's box), and it just did not sound 'right' with tremolo. Then I tuned them all dry (MMM 3 voice) and suddenly the box had a lot more power - they closed in to unison very easily, and 'worked together'.
I find that the same goes when just one of the reeds are less than perfect quality - has been retuned from another note, is not set right in the slot, valve not opening correct etc. etc. The 'strong' reed will have a tendency to suck in the weak reed. This is just what I've experienced - perhaps someone with more theoretic knowledge or practial experience has another point or two on this matter?