In SynEdit the caret always has to be at one end of the selection. And that is always the end of the selection that gets changed.
(One end of the selection gets fixed (to where the caret originally was), the other end follows the caret (the mouse moves the caret))
You may be able to set them separate by code. But then (if that works) you have to write your own mouse button/move handler.
Actually the caret can move away from the selection, if you enable persistent selection. But the caret is still used to create the selection.
But if you remove any mouseaction from the button, and do your own code, it MIGHT be possible.