If you want two Wraithguards here is how to do it.  First of all
you need to follow the story up until Lord Vivec gives you the first
one.  Here comes the hard part.  You then need to kill Lord
Vivec.  After killing him the game will tell you how you have
created a doomed world and who cares, this is the only time you need
to confront him and with that said you just take the Mysterious Dwemer
Artifact that is own his dead body.  Then you take it to the
Corprusarium in Tel Fyr and speak to Yagrum Bagarn.  He will tell
you to get the following two book.  Kagrenac's Planbook in
Kagrenac's Library inside of Tureynulal and Kagrenac's Journal in the
citadel Endusal.  Just be sure when you get the two book you kill
the vampires and take the pendants that they wear.  Then take the
books back to Yagrum and have him activate Wraithguard.  Odd how
the one Vivec gives you is for your Right Hand and the one Yagrum
Activates is for your left.  Regardless you now have two
Wraithguards.