Colliding flame-fronts really isn't a big problem. Early tests at muliple-plug heads suffered from some simple assumptions- 1) same plug and 2) same timing. The burn time of a dual-plug head is theoretically HALF that of a normal head (in reality, it's hard to hit that, but figure at least a quarter-to-half, if the plug is located well). This rises chamber pressure more rapidly.
Most make the mistake of running the same (hot) plug... when the better move is to run a cooler plug, to get heat away from the electrode tip. The other mistake is running too much ignition advance... between hot plugs and too much advance, you get preignition (and melted plugs).
So locate the plugs well (both of 'em, usually), drop back a few points in plug temp, and drop back 6 degrees or so of advance, and tune for maximum whomp.
The multiple-plug concept really has most effect in a shrouded, contorted chamber (like a flathead). Doesn't do much for a hemispherical or spheroidal, simply because those heads already have good flow, but the multiple-plug arrangement has been very proven in engines- especially in the 2.5l Ford Ranger four from '89 on.
I've been down this road with a Model A Ford and a Continental Red Seal head. Wish I could do it with my Chevy inline six, but there's no room.
Here's some good links on the subject.
http://www.visi.com/~moperfserv/2plug.htm
here's a link on flathead performance tests in the Model A Ford (a classic flathead design).
http://users.aol.com/gmaclaren/dyno.html
