Where did folder "merge" or "keep both" go in OS X 10.9 Mavericks?
OK. I found out myself... Luckily Apple did not remove the Finder Merge functionality with Mavericks. But sometimes it is not offered to the user. But let me explain:

Supose you have a folder named TARGET with a subfolder TARGET/SomeStuff. And you have a SomeStuff folder somewhere else. Now drag this second SomeStuff folder to the TARGET folder (without any modifier keys pressed) then the Finder will presented you the following dialog (Stop and Replace):



So, no merge here.

Now - if you instead press the Alt key while you drop the SomeStuff folder to the TARGET folder there are two possibilities:

1.) If the files in the two SomeStuff Folder have completely different (not overlapping) contents. In other words: no duplicate filenames. Then finder will give you these options (Stop, Merge &amp; Replace All):



So, here you can merge. And the result will be a TARGET/SomeStuff folder with the content of both SomeStuff folders.

2.) But here comes what is sometimes confusing (and actually made me think merging is gone in Mavericks): if there exists at least one filename that is contained in both SomeStuff folders, then merging is not possible. Then the finder will offer you these options (Don't Replace, Stop, Replace):



So, no merge possible, here too!