As far as I can see you can't - Excel is not paying any attention to where
your mouse clicks unless and until it gets the focus. So the first click can
only give it focus, the second will then click in a particular place.

You could always ALT-TAB to Excel and then click. Still two operations but
might make more sense depending on what you are doing.
--
Adam Vero
MCP, MOS Master, MLSE, CWNA
http://veroblog.wordpress.com
http://www.meteorit.co.uk