Proper connection of a two-way circuit is (sorry about the ASCII sketch)
sw1 sw2 load
hot ____/ ______()_____ neutral
... two single-pole, double-throw switches back to back. When both are switched one way (up in this case), one of the wires between the switches (the travellers) carries current to the load. When both are switched the other way, the other wire between the switches carries current. When they disagree, as shown here, power is applied to one of the wires between the switches but the other switch is connected to the other wire and no power flows.
The details of how that's actually laid out in the house depends on the relative positions of switches and load. One approach is to bring three wires up to switch 1 (hot, neutral, safety ground), run four wires to switch 2 (top hot, bottom hot, neutral, and ground), then run three wires (output of switch 2, neutral, and ground) from there to the light. I've also seen variations where rather than running neutral between the boxes they ran the output of switch 2 back to switch 1's box and then wired to the load from there. I'm not sure whether both approaches meet code; I do think that either needs to be flagged to death so the next person to open the box is certain of what they're looking at.
One of the travellers, of course, will always be hot. If that's what you mean by "common", this isn't surprising and the answer is Don't Do That; touching any live circuit is a Bad Idea. If by "common" you mean neutral, there are other and larger problems...