@palemantle, check out the Tunnels thread. There have been several combats in there, including the goblins, the ghoul, more goblins, and the not-mummy. (Oddly enough, I think we've avoided the derro.)
In those examples, they are usually very close in (after all, tunnels) and everyone pretty much just does the two attacks. However, the primary other options for one of those actions are: focus, evade, and move.
Focus means that the following action will succeed on a 4, 5, or 6 instead of just a 5 or 6. Statistically, in combat you are pretty much always better off just making two attacks. The border cases are very rare.
Evade gives you a chance to avoid a hit. If you have the Shield Master trait, it is likely you will use this frequently. On the other hand, a duende will never use it because they basically are evading at all times for free. So, if you make only one attack and evade, then for each hit that turn you can roll a single die and on a 5 or 6 you will not take any damage from that hit.
Move lets you move however the GM defines it. I believe here that basically means moving from one zone to another. To run away, you are pretty much required to move for both actions.
Does any of that help?