You're right, once the lights are on, I can control them, but I can't turn them back on once they're off.
I've never been able to turn them on remotely, even before I changed the switch for the XPT.
I removed the switch, which was on the hot side, used a Marr connector to join those wires & pigtail in the black wire from the XPT. I then pigtailed the white wire into the white electrical wires.
An electrical wire runs from the XPT to the dual-gang box with the 2 XPD3s. The XPD3s are pigtailed into the source wire & the wires running to the pot lights (2 75 watt flood lamps per XPD3).
I don't think the XPD3s went bad, I'm just uncertain whether they're suitable for the task.
The XPD3's should be fully capable of controlling your can lights.
I'm still trying to understand your wiring configuration. Is this an
"old" house (wiring from a previous owner)?
It sounds like your XPT location (your old on/off mechanical switch) was intended to be a 3-way installation (lights controlled from two locations). Do you have extra red wires in your boxes? This can be a quick tip that you're dealing with a 3 way installation. The red lead is part of a 3 wire bundle that is run between the two switches (traveler wire-see diagram).
Unfortunately, many times the installer will run two sets of 2 wire cables between the two switches (4 wires - 2 balck and 2 white). This can make it very difficult to determine which wires are connected where.
What I'm thinking is that somehow your XP3D switches are wired in series rather than parallel. When you turn off both switches they can no longer receive X10. If you don't understand the series vs parallel let me know. I can try to provide a diagram tomorrow (I'm using my A/V pc and it's graphic arts capabilities are nil).
If this is the case, you'll need to trace out where your power is entering the circuit, and which wires connect to the loads (lamps). There are a whole host of possible configurations, including having the power enter through your lamp fixtures (I really hate it when they do this).
Bottom line, you'll need a volt meter and you'll be working with live circuits. If you don't have a meter, or aren't comfortable with having live wires sticking out of your electrical boxes (I'm always worried about the kids), by all means call an electrician.
The Boiler