Starfall:
Generally: Use when both are up.
Specifically: Find out, for your gear level, here :
https://docs.google.com/spreadsheets...uOk/edit#gid=0
Empowerment:
Generally: Use for both eclipses
Specifically: If you have 4p and euphoria, then it's obviously good to use it for solar as well since faster casts mean more casts and faster CA cd reduction. Before 4p you can keep charges for Lunar only but never cap them. Non-euphoria, use them in both as eclipses last too long and chance to get charge caped is higher.