I have two iPhone Events folders in iPhoto. Why? How combine?

There is no "Event folder" - there are events and there are folders - folders only contain other folders or albums - event are the basic automatic organization - the easiest way to move photos between events is to flag the photos in one event and then select the destination event and use the add flagged photos to the select event command

LN

Do you mean two iPhoto Events folders?

If so click on the one folder (1) you want to move into the other folder (2) and select all of the albums in the large window on the right. Drag them onto the icon of folder 2 that you want to keep and release. That will move all of the albums in folder 1 into folder 2.

Now that folder 1 is empty you can delete it by Control (right) - clicking on the folder and select Delete Folder from the contextual menu.