two single coil pickups in series is similar to a humbucker - you get a fatter sound. A humbucker is usually wired up in series (though it can be changed to parallel). Single coil pickups are generally wired in parallel - like a traditional strat or tele wiring.
http://www.musicplayers.com/tutorials/guitars/2006/0906_Pickups101.php
Series versus Parallel Wiring
In guitar wiring, series and parallel methods are talked about most often when discussing humbucking or dual-coil pickups. When two coils are wired in series, they are wired in such a way that the end of one coil is connected to the beginning of the other so that the signal travels in one path along the full length of both coils.
When two coils are wired in parallel, the signal is split in two (not always equally, dependent on coil resistance) before entering the coils. Each half of the signal travels along both coil paths at the same time (in parallel) and is combined at the end of each coil.
Imagine that six people in your band represent your instrument signal, and the individual coils of a humbucker are represented by two roadways. The start of the signal chain is your rehearsal space and the end of the chain is the venue where you’ll be gigging tonight. The resistance of each coil (roadway) will determine how many people can fit into each car.
In series wiring, everyone in your band jumps into one car, drives the entire length of the first roadway, then drives the entire length of the second roadway and arrives at the gig.
In parallel wiring, the band splits up into two cars and each car takes a different roadway to the gig. Despite any differences in number of passengers, vehicle, or length of individual roadways, both cars arrive at the gig at the same time, where the band then regroups together.
Both methods of wiring can be hum-canceling and both can be in phase or out of phase.
Series wiring is the common way in which most humbuckers are wired and yields the strongest signal, as the signal from each of its two single coils are added together to form A + B.
Parallel wiring is used most often for obtaining pseudo-single coil tones without the hum. The signal is an approximate average between the strength of each of the two coils. The actual method for calculating this is to multiply A x B and divide the result by A + B.