I have never heard of that particular explanation before harley.
The birthrate of twins is on the increase, but is still less than 3% of births.
Lefthandedness is on the increase, from a base persistent rate of around 15% in the general population.
Wiki has an interesting page on left handedness;
http://en.wikipedia.org/wiki/Left-handed
And there is some thought that it might be genetic
http://www.left-handed.com/forum/viewtopic.php ?t=87
Left handedness is up to 3x more common in men than in women, and some researchers have hypothesised that Left handers were exposed to higher rates of testosterone pre birth.