That's a 3-way circuit and must be wired differently. You have three basic choices:
1. Put in an X10 switch in one location and deactive the switch (wire around it) in the other location.
2. Put in an X10 3-way switch in one location and an X10 slave switch in the other.
3. Put in an X10 switch in one location and an in-wall keypad transmitter in the other.
I've drawn up schematics for the 2nd and 3rd options and have them posted online at
[Link: ourworld.compuserve.com] My schematics assume the use of the PCS SmartSwitch line of X10 switches but the same principal applies regardless of the switch you use.