Dusk and Dawn
+1 Enhancement
Weapon property
Choose two energy types (fire, acid, cold or electric). During the day, the weapon deals an extra 1D6 points of damage of one energy type chosen, while at night it deals 1D6 points of damage of the other energy type chosen. You must choose which type is active at day and which at night when the weapon is enchanted, this choice cannot be changed later. This ability functions even underground, regardless of whether or not you can tell what time it actually is. Spells such as Daylight, Darkness, etc. do not effect the weapon's energy type, only the actual time of day.
Dusk and Dawn, Greater
+1 Synergy Enhancement
Prerequisite: Dusk and Dawn
Weapon Property
The weapon functions as a Dusk and Dawn weapon, except you can switch between the chosen energy types as a standard action, regardless of the time of day. However, only one energy type is active at a time. In addition, during the twilight hours of dusk and dawn, the weapon is able to use both energy types at the same time, effectively dealing an additional +2D6 points of damage (+1D6 fire, and +1D6 acid, for example.)
And then with Jujun ruling that you can't use Suppress Requirement, it should work out to a flavorful balanced pair of abilities, since most people would just pay for the +2 to make the weapon basically Flaming Acid, or whatever.
EDIT: If the synergy ability seems too weak, you can always make it deal D8's during dusk and dawn. Slightly more powerful than the standard, but limited by time of day to help balance it.