Multiple devices and desktops works fine with opensync, due to the way it's engineered. I've tested a group containing my WM6 test device, a Nokia phone, KDE and Evolution, and this four-way sync works fine. I believe it would work correctly with your example case, too.
Palm isn't mentioned simply as I don't have a Palm test device so I cannot be confident that the Palm support in OpenSync works, or document the exact procedure :(. I'll try and get hold of a test device, but if I can't, you'll have to just do it yourself; it shouldn't be too hard, install the appropriate libopensync-plugin-* packages, and kdepim-kitchensync, and try it.
If anyone's got an old Palm device gathering dust that they wouldn't mind me putting to a good use

, let me know. I think I would need it to be a USB-connected one, but other than that, anything should do.