Use of two hands on both sides is still not seen in large numbers...but, is growing in both the juniors and the pros. As a pro who uses the two-handed forehand as both a learning tool as well as recognizing that the use of two hands on both sides does not prohibit a player from reaching world-class status, I have seen it's value over the past 18 years I have taught the stroke.
Here is an interesting point: On the women's tour, there are nearly an equal number of two-handed forehand players in the top 100 as there are one-handed backhand players. (At last count, it was 6 Two-handed forehanders, 8one-handed backhanders.)
Granted, a one-handed anything can generate greater whip and, hence, racquet head speed. However, if such speed can't be controlled, it is essentially useless on the tennis court. We see this same concept on the golf course and the baseball fields. Control is the key especially in tennis and golf.
Thus, players who do use two-hands tend to have more stability and repetitive swing components, or are able to create a more reliable swing pattern...especially early on.
In my book, I teach all three 'two-handed forehand' methods; Seles' version (which is the more dominant one), Gambil/Sluiter's which the hands switch (basically a backhand on each side) and Gene Mayer (from the 70's) who kept his dominant hand as the top hand.
We find players develop certain aspects of the 'modern' forehand faster with the two-handed forehand. But, it isn't for everyone and we usually explore the one-hander quickly unless a player is really liking and finding the two-handed forehand a weapon for them.
If interested, look up my articles at
www.tennisone.com on the two-handed forehand. My articles have been translated too, into Dutch and German for those countries tennis magazines as well.
Hope this helps.