Silhouette's ability is not Before Combat. It can be used during the Ranged, Melee or Magic Phase (even though it has no purpose in the Magic Phase).
Remember that Before Combat is not a "phase", so she cannot use her ability in Before Combat.
If you spend 2 E after attacking in Ranged, the 2 E after attacking in Melee, you can attack in all Phases for that combat round.
The difference between cards that you can activate and abilities you can use "Before Combat" is not that difficult to understand:
1) you can use each of your Hero's or Allies' Before Combat abilities once
2) you can activate a single "activate-to-use" Item that has an ability in Before Combat
Besides this ruling given in the basic Rulebook, the FAQ says that A card may be used multiple times for the same effect unless the card states otherwise. This is the reason why Silhouette's ability can be used multiple times and Mad Carthos or Astarra cannot (they're Before Combat abilities).