Based on the fact that the core has changed, if developers were expected to write for WP7.8 then there would be little reason to change it to a Windows 8 shared core. At this stage MS have confirmed native code support is NOT supported by WP7.8 so no you cannot write code in a language both WP7.8 and WP8 understand. Some (I expect a lot) will write native code for WP8 and WP7.8 will be sh*t out of luck. If you believe otherwise then good luck.