I'm not exactly sure. I'm not really up to speed on the latest techniques for normal mapping but the client wants both tangent and OS maps. I believe it's for cryengine.
This is why you should set up your model to bake nicely in one go. If your workflow involves a lot of tweaking or painting over the normal map or any of this stuff its a big waste of time if you have to rebake, make revisions, etc.
Hard to say without knowing exactly what problems you're running into. I do a lot of hard surface stuff, and rarely ever find the need to edit normal map errors out in photoshop. Most issues can be solved by good planing, explode baking and understanding how your mesh normals project out from your lowpoly mesh.
Generally a bit of extra time making sure your mesh bakes well in one shot (and with experience you learn to avoid doing certain things that bake poorly) will save a lot of time dealing with "fixing" things, which is compounded with any rework.
Replies
What is your purpose for going from tangent to OS?
More here: http://www.polycount.com/forum/showthread.php?t=81154
Maybe here as well: http://www.polycount.com/forum/showthread.php?t=107196
Generally a bit of extra time making sure your mesh bakes well in one shot (and with experience you learn to avoid doing certain things that bake poorly) will save a lot of time dealing with "fixing" things, which is compounded with any rework.
Checking those threads out