I'm a bit confused. Are you saying you want to use X10 for this project and just change out one of the pair of switches for your lights?
Do you just want to put in a non X10 dimmer where one of the switches is? I don't think a single X10 switch could work as some current must flow through the switch at all times to allow the X10 signal. The other switch would effectively become a master off switch.
A conventional 3-way light is wired like this. The hot line goes to one switch and two wires connect the two switches.
___________________ light
Hot_________/ ___________________ \_______?____
AC |
Neutral________________________________________|
When both swiches are up current can flow through the upper line. When both switches are down current can flow through the lower line. If one is up and one is down, there can be no flow of current. This is why both switches must be changed.
Just be glad your house isn't wired with 4-way switches. Anyone else out there ever seen a 4-way switch?