The first value sets each particle's opacity on creation. The second value sets the range in which its opacity can change after it's been created per second. One such usage lets the particles twinkle.
All of the particle effect values follow that same format, by the way: the top values are for on creation, and the bottom values are for during its lifetime.
There's no way I can think of to do what you want, though. The Fade behaviour only applies to the whole particle object (the emitter, basically), so it'll fire off once and be done with it. It doesn't affect the individual particles
To sort-of get what you want, I suggest setting the particle opacity and starting size very low (say, 1 and ten), then setting the growth rate quite high (about 80-100). It's not perfect, but it could work for something like a jet of flame or smoke.
Your other option is to create your own emitter using the Bullet behaviour. That way you have direct control over the sprite animations for opacity, or just use the Fade behaviour. It'll require a bit of messing around to get the same functionality as a particle emitter, but it's effectively the same thing (just with a much higher system load).Example capx (r135)