i finished the game 7 times on nightmare difficulty and these are my suggestions.(these ladies can learn/use more spells,indeed.but i suggest the spells which is REALLY NEEDED)

for wynne:

1- Fire Ball

2- Haste

3- Mana Clash

4- Animated Dead

and morrigan

1- Cone of Cold

2- Glyph of Paralysis

3- Crushing Prison

4- Death Hex

5- Blood Wound(Of Blood Mage)

6- Sleep