Generosity

It was temporary because it couldn't last very long. 

The fusion was an extremely crude combination of their energy, and the amount of energy that Liz'An would lose in the process was so huge that the only reason that they could even fuse temporarily this way was because Liz'An's energy was almost endless. 

She could afford the energy loss!

That's why it was her who sent her energy into Wywy, and it was Wywy who ended up casting the spells. 

GRAAAAAAAAAAHHHHHHHHHHHHHHHHHHHHH!!!

Wywy roared out in exhilaration as he felt Liz'An's arcane energy fill him to the brim. 

"NOW THIS IS POWER!!!" He thought maniacally. He opened his massive maw and breathed out an utterly fearsome dragon breath that instantly melted thousands of unlucky undead caught in its area of effect. 

"Get a grip of yourself." Liz'An sternly reminded Wywy. "Our real enemies are those two giant skeleton monsters."

"Yes, Mistress!" Wywy roared in excitement.