If you don't have software that sends the keypresses to both WoW clients (software such as Keyclone and Octopus) then there is no way for the keys to get there.

However, with a G15, you can do a very simple dual box by mapping the G keys to "ALT-TAB x ALT-TAB". This gives you one key to press that will switch clients, press a key (x in this example) then switch back to the other client. That's what I did when I first started out. Since then, I've moved to software that lets me multibox. Check the Wiki link above and read the articles in Getting Started for other ways you can set yourself up.