Ok.. What are the effects of loosing hum bucking when the single coils are together? Sorry being a bit thick.
"Humbucking" actually means, as you could guess, "Bucking the Hum" or reducing/eliminating hum.
Single coils pick up the 50/60 cycle hum that comes from the electrical mains that your gear is plugged into. I'm sure you've noticed that 'buzzzzz' you get from single coils when you are close to an amp, or any electrical device.
Humbuckers were designed to reduce this effect by using a reverse polarity magnet, and reverse coil design. One half has the north side up, with the copper wound either clockwise, or counterclockwise, and the other half (coil) is the exact opposite. They are then connected in series (one coil is run through the other)
When a humbucker is properly set up, one coil picks up the hum in a 'positive' manner, while the other is inverted, or 'negative', add them together and they cancel each other out.
This also has the effect of boosting the output of the signal (that's why humbuckers are louder, usually) but as a trade off, you lose some high end, giving them their 'warm' characteristic.
When you split the humbucker to a single coil, and pair it with another single coil, as long as they are reversed in relationship to one another you still get some of the hum canceling effect.
Since they are in parallel instead of series though, you get that 'Fender Quack' instead of the warm, louder sound.
If anything isn't right, coils wound in the same direction, and/or the magnet polarity is the same, you lose the hum canceling effect since the electrical properties simply aren't there to do it.
Custom pickup designers realized that you could alter the tone buy 'mis-matching' the coils/magnets (more or less winds of wire and or weaker/stronger magnets on one coil). That's why some humbuckers tone is altered if you reverse them.
They aren't as good at 'bucking the hum' but with most modern equipment, even the single coils aren't as noisy as they used to be, so it's not a big deal.
Did you get all that?