If you are going to use two lightposts, one for the main line and one for the siding, then with two pair PF-lights you can set this up. Have from PF-lights A go one to the red on the siding and the other led to the green on the main line and from PF-lights set B one LED to green on the siding and red on the mainline. You'd need two switches though to turn the LED's on, turn A on turn B off and visa versa. If you remove the rectifier from the PF-lights you can use the reversal switch, to switch between PF-light set A or B with just the one switch.
The best looking solution though would be to remove the rectifier from the PF-lights (I believe it's situated in the 2x2 plate around the middle of the wire) and at the same time switch the left and right wire connection of 1 of the LED's. So for this solution I think you only need to take apart the 2x2 plate in the middle of one of the wires.