Yes I tend to agree. You see a lot of the girls actually grew up playing on a combination of grass and dirt courts which are even more slippery to play on than the European clay courts. I grew up on them myself. So it explains why so many could handle the grass and clay so well. And it was the same for the men - they generally had a good handle on both surfaces eg. Rosewall, Laver, Roche spring to mind.
Yes you are right Iain, I think the Aussies probably led the way in terms of double handed play ... there was Vivian McGrath, John Bromwich (double handed off both sides), John Brown (I think that was his name ... made a Wimbledon final), and Jan Lehane the first of the women that I am aware of.
Lehane played her double hander very close to her body so it really would have restricted her somewhat and i think it was probably her undoing on grass to a degree. I also think it was a case of the baseliner develops their game quicker - they don't have as much to refine, but the all court player takes longer to develop. So although she had the wins over Court in the juniors, once Court developed her all court game (rather quickly mind you) she then turned the tables and started to dominate Lehane. Mind you, Turner posed more opposition on clay for Margaret, but as we know, Lesley was one of the greats on clay in the early to mid 60s.
Geoff Brown was the double-hander you're thinking of - John Brown came a generation later, and became better known as a tennis promoter.
I ballboyed for Jan Lehane at the Australian championships in 1961, and watched her play often. "Tenacious" was the word for her. Her game, as I recall, was really pretty similar to the double-handers we've seen in the past generation: fierce driving groundstrokes into the back corners of the court, not much variety or artistry, but quick on her feet, and as Adrian Quist said, played with a tigerish intensity. Against Margaret, though, that just wasn't enough. Margaret had such big weapons - that serve, superb volleys, and powerful, audacious groundstrokes - that she just blew Jan and pretty well everyone else off the court.
Re Australians on clay: in Melbourne at least, we grew up on clay, or en-tout-cas, as we called it, so it's no surprise that lots of Melbourne players got their best results on clay: Merv Rose, Peter McNamara, Nicole Provis Bradtke, to name a few.