Not sure why you want to merge the two tags...
As you say, they're related in that they aim to accomplish the same goal. But the implementations are entirely different, and so are the behaviors. To quote the fakeroot-ng homepage:
On the functional level, there are many differences between fakeroot and fakeroot-ng. Some of these differences are a result of the different technologies used by the two programs, and some are more derivatives of the above. The differences cover such areas as default ownership of unknown files, handling of SUID executables and chroot support. The differences are covered in more depth in the fakeroot vs fakeroot-ng comparison page.
Presumably, then, the questions could be distinct as well, and the experts as well.
Two separate tags seem entirely appropriate. Well, at least as well as can be determined from the 0 questions that actually are so tagged.