I figure the most intuitive way would be if WIN-single-click would bring up the start screen appropriate to your current mode and WIN-double-click should bring up the screen for the respective other screen:
in Metro mode:
- single click of WIN key brings up the Metro start screen
- double click switches to desktop mode
in Desktop mode:
- single click brings up the Start8 start menu
- double click brings up the Metro start screen
Here's hoping that a future version could incorporate that behavior 