What's the best way to link two (or more) LEGO Power Functions (PF) train motors together? Last Christmas I built an extended Emerald Night train to run around the tree. To get it to work I had to add a second Power Functions motor. I tuned both IR receivers to the same frequency and slowly applied power by keeping the IR remote within the line of sight of both receivers at the front and the back. The major disadvantage to this was it was difficult to sync the two engines because line of sight was not assured.

I'd like to build a larger train this year and would like to find a better way to link the motors. I've think that putting one motor in the back helps with weight distribution, although this idea may be flawed.

What the best way to link multiple Power Functions motors together without having to rely on multiple IR receivers? 



Below are some photos that provide more detail about the train in question.

All photos http://www.flickr.com/photos/craigmoliver/sets/72157628313822041/

Relevant photos

Front engine, standard Emerald Night (10194-1):



Rear power car, custom (MOC):
If the motors are far apart, you could simply connect them both to the same receiver using some extension cables. To the best of my knowledge, there is no problem connecting two motors on the same input of one IR receiver.

If you're using Technic motors instead of train motors, as Zhaph said, you'll probably want to have them coupled through a differential so that no motor can stress the other. That's the standard way of doing it in any Technic creation, and since it effectively adds the two input speeds (from the motors), it means you can create some variation in speed very easily.

I did that to power a PF-modified 7740 engine with two medium motors when the only remote available was the simple on/off one: one motor had its speed reduced to 1/3, so the range of available speeds where 4/3 (both motors on), 3/3 (regular motor on, 1/3 motor off), 2/3 (regular motor forward, 1/3 motor reverse), 1/3 (1/3 motor only), 0, and the same in reverse. Was awfully complicated to use the remote to go from one speed to the next, though. And then LEGO produced a speed-control remote and I didn't look back to that idea again.)