Lars,
I don't know much myself -- like I said, I expected the two switches to be able to associate directly with each other, but seems they need a controller for that. I THINK that once associated, they talk directly to each other, but don't know for sure. Here is the info from the Linear WT00Z "Z-Wave 3-Way Wall Accessory Switch", direct from the papers in the box:
"Associating Z-Wave devices to control from the WT00Z-1
Refer to your controllers instructions to find out how to associate Z-Wave devices. You can associate another Z-Wave device into any or all of the 4 available groups in the WT00Z-1."
It then goes on to describe how to turn on/off or dim all the devices in each group (group1: the switch acts just like their normal dimmer - tap the top to turn it on, tap the bottom to turn it off, press and hold the top to make brighter, press and hold bottom to make dimmer. group2: like group1 but one extra tap first. group3: like group 2 but TWO extra taps, etc).
So sounds like the controller should be able to figure out that the switch has several groups, then somehow add various devices to each group.
You now know everything I know...