I want to play a 600 point mordor army with two ringwraith on fellbeasts. I chose the undying and have no idea about who to take as second wraith. But here is my idea for the list:
Warband 1 10/12
The Undying with Fell Beast;
2 Orc Warrior with Shield;
4 Orc Warrior with Spear;
3 Morannon Orc with Shield;
1 Morannon Orc with Shield; Spear;
----
Warband 2 10/12
Second ringwraith with Fell Beast;
2 Orc Warrior with Shield;
4 Orc Warrior with Spear;
4 Morannon Orc with Shield;
----
Warband 3 9/12
Mordor Orc Taskmaster
2 Orc Warrior with Shield;
3 Orc Warrior with Spear;
3 Morannon Orc with Shield;
1 Morannon Orc with Shield; Spear;
_________________________________________________
600 points; 6 might; 32 models; 2 fellbeasts!
Do you think two fellbeasts are to many at 600 points? I really like the idea of having another ringwraith beside the undying to get back many will points and two spells per turn and two monsters really seem funny. Do you think the morannon/normal orc mix is ok?

Difficult to say,
Your problem is that the oponent only needs to kill 17 orcs to break you.
So 2 fellbeasts is maybe too much of a points sink.
I would take the dark marshall on horse. gives you more support for your troops (6" banner) And he is free to use his will as he would not be fighting. Perfect support wraith for the undying on FB
Also, I would not take the taskmaster. In order to benefit from him, your wraiths need to be close all the time, and that limits their effectiveness a lot. The best thing of a fellbeast is that it can pick its fights with 12" movement. I would just use a cheaper captain with 3 might (grisnakh, gorbag, shagrat) or a shaman like kardush.

The task master is fine IMO as he can effect his own heroics. I'd vote Khamul on Fell Beast. Simply use The 2FB's to wreak havoc on your opponents. And "only killing 17 orcs" is no easy task with 2 Nazgul throwing your troops around like rag-dolls.

To be honest a banner might be a good investment too. Obviously you don't want to drop your numbers too much but the reroll could up your survivability for the troops. Or a Shaman. Your Orcs really just need focus on survival and buying time for the Nazgul

_________________
Commission Painting @FB http://www.facebook.com/squyrepainting
Commission Customers include:
GBHL Youtube Channel
MiniWargaming

Of cause I'd like to have a banner and I would take a shamane, too. But I have only 600 points to spend
32 models aren't a lot and most of them are defense 4 or 5. I took the morannon orcs to have a bit more stamina. I'm thinking about taking a few Black Numenorers, too. With high defense, fightvalue, terror and maybe shielding they would add a lot stamina. But I only have 3, yet.
I decided to take the taskmaster, because in my opinion he is worth the 20 points more, if he only saves two might points with his special rule. A shamane could be a good choice, too. But I'm a little bit afraid I have not enough models to be sure he does not get attacked.
I think Khamul fits good, he has the possibility to get more will to, and can get fight 6, the problem is the higher dicerolls needed to cast the spells (and the undying only gets will points back, if he is succesfull casting a spell).
I think the Knight of Umbar is a really good one, too. He only looses will, when he loose a combat and has one extra might point, but he also isn't that good casting spells.
The dark Marhsall is an option, but only 12 will points. This is difficult