When Loki gained the ability to use inner energy, the first thing that came to his mind at that time was if it was possible to merge his mana with his inner energy. The two powers came from completely different sources and even though they had some similarities they worked differently.
Inner energy was produced by one's body and like any body part, it could be strengthened with proper training. The stronger one's body becomes the greater the inner energy it could store.
On the other hand, mana was the spiritual inner energy produced by the world. To strengthen one's mana supply one needs to become more attuned to it. The greater one's understanding of mana the faster one's mana supply increases. It also helps if you were at a spot that has concentrated mana.
