The following applies to North American electrical systems....
Two phase is not the correct terminology and sometimes, the terms "split phase" or "split leg" is used. The 220 power that we are familiar with is what comes into our home service panels and it consists of one neutral and two "hot wires". Somewhere in the neighborhood, there is a transformer with high voltage coming into it. They put a winding next to the core and it induces 220 volts into the winding. The two wires from that winding are the two hot wires that enter your house. If you measure the voltage between those two hot wires, it will read 220 volts. In the middle of that winding, they put a "center-tap" which is just a wire that connect to the center of the winding. That wire is the neutral that is brought into your house. If you measure between the neutral wire and any hot wire, the voltage is 110 but, one with respect to the other is 180[SUP]o[/SUP] out of phase. Without a significant description of phasor mathematics and something called "complex numbers", it would be very hard for me to describe in other terms why the phase change occurs but, suffice it to say, it does indeed occur.
BTW, the terms 110, 120, 220 and 240 are a little confusing. 110 and 120 are the same really; it's just that the power company is allowed to vary the voltage under different circumstances and it could be somewhere in that range. The frequency (60 Hz in North America) will not change and if it ever does, there are serious, serious problems going on in at the generation facility -to the extent you will probably see a bright flash coming from the facility. -Basically, it does not vary and if it does, it happens simultaneously all through North America. The entire system is synchronized.
Ray