Can I combine two actions into one box, and if so when am I allowed to and how do I do that?

Yes you may combine 2 action boxes at any time you wish.  

First, the two Actions must be related in some way, i.e. Acrobatics and Close Combat; Invention and Technology.  

Second, the AN of the new action is the lowest of the 2 you combined.  This means that if you combine Acrobatics AN 5 and Close Combat AN 3, your new actions AN would be 3.

Third, the new action may be renamed, if you wish, and the old actions become options of this one, allowing you do the two actions from this one box.  Any specialties are carried as well.

For more information about combining 2 action boxes together see this FAQ