defaults write "Apple Global Domain" AppleScrollBarVariant DoubleBothYou then need to logout and login (or, perhaps, simply force quit the Finder) to see the effect ... but once you've done so, you should have double-scroll arrows at both ends of your scroll bars.
I'm not sure which combination of applications (Cocoa, Carbon, Java) this applies to, but it works for certain in the Finder and a couple of Carbon applications I quickly tested. To return to the normal mode, use:
defaults write "Apple Global Domain" AppleScrollBarVariant Single(or you could just open the General pane in the System Prefs and check "At top and bottom") or
defaults write "Apple Global Domain" AppleScrollBarVariant DoubleMaxto put them together only at the bottom (again, this is equivalent to clicking "Together" in the General prefs panel for scroll arrows).

