#1
Looking on the Dimarzio website, and in the descriptions, there's an "output" number. What does it mean by output? Am I better to get two pickups of the same output or does it not matter?
#2
teh out put is basicaly how hard the pickups will hit the front of your amp with a signal. the lower the output, the lower the signal is that reaches your amp ( these seem to stay cleaner at higher volumes). High output puckups hit your amp with more power and will actually overdrive your amp to a certain extent.

With modern amps that have so much gain on tap in the pre-amp, you really don't need high output pickups to get that modern hi-gain tone.

In the end both low and hi output pickups have very good points about them, it is up to you to figure out which are good for you.
2002 PRS CE22
2013 G&L ASAT Deluxe
2009 Epiphone G-400 (SH-4)
Marshall JCM2000 DSL100
Krank 1980 Jr 20watt
Krank Rev 4x12 (eminence V12)
GFS Greenie/Digitech Bad Monkey
Morley Bad Horsie 2
MXR Smart Gate
#3
you don't want a pickup in the neck to have as much output as the bridge (typically) check out some pickups that have both neck and bridge versions available and you will see a diffrence in their output levels.

Just for examlpe Seymour Duncan pairs the SH-2n (neck position) with the SH-4 in the bridge position, and the SH-4 has much more output than the SH-2n. But those pickups tend to complement each other.
2002 PRS CE22
2013 G&L ASAT Deluxe
2009 Epiphone G-400 (SH-4)
Marshall JCM2000 DSL100
Krank 1980 Jr 20watt
Krank Rev 4x12 (eminence V12)
GFS Greenie/Digitech Bad Monkey
Morley Bad Horsie 2
MXR Smart Gate