Doh! Too slow on the job this time.
I was following up Greg's post with one about the extra plug that Chris just mentioned.
Here's what I wrote, in case it adds anything useful:
If you're just running stock cars, rather than heavily modded cars that need lots of power, you can get by with one PSU. You need to plug the other hole with a properly fitting plug, though. If you have another device that fits the socket, plug it in, but do NOT NOT NOT plug the other end into the wall power. We only want a dead plug in that jack to tell the base that it's ok to use both lanes. Ideally, you would cut off the plug from whatever it used to be on, to prevent any mistakes.
But, as Greg pointed out, the best option is to get another PSU. It will give you more power for two analog cars, not to mention all 6 digital cars (or more, if you set up more than one as pacer on the same ID ;-). On top of that, there would be no worries about warranty issues, because you'd be following the manufacturer's guidelines.
For more detail on Analog mode, please see my video: