This is a long explanation that begins with the premises to two separate characters that are actually one. The background story is included to explain the circumstances of the characters more clearly.
In a campaign in the past, I had two characters. One was a manga-obsessed girl with the ability to manifest light to heal and even turn into holy fire. She was cheerful and didn't have a clue. The other was set up like a Shinigami knockoff for reasons you will later understand; she was the opposite — her power was the power of death. Those who approached her felt the cold and despair of Death itself.
Here comes the fun part: they were "sisters." Lina the kind-hearted clueless manga freak and Selene the stoic, silent type. Selene has a secret, though… she isn't supposed to exist...
It all started about the start of the first session. Lina Grey was 11 years old and had gotten lost in a forest in somewhere, some country, Earth. She had fallen down a well and found an old ruin with a pedestal containing a pool. This pool was known as the Pool of Awakening.
Long-story-short, the Pool of Awakening caused her powers to manifest. (It didn't give her them; she had them dormant.) The strain of her dual-natured powers of Life and Death was too much, causing her to split into two different people, each taking an aspect of her power with them. One was, of course, Lina; the other Selene. The stress wiped Lina's memory, but Selene knew everything.
Long-story-short-again, they became heroes after a few years of Lina relearning everything about her life that she forgot (exept for the incident) and Selene posing as her sister.
Since I'm starting a new campaign involving the heros from the last one, is there a way to combine them into one character with two personalities, one manifesting if a certain condition is met and turning back when another is met? The power level of each character is 10. The conditions for changing to Selene are either taking sufficient damage or being close to a Pool of Awakening. Changing back to Lina is a little harder; Selene must defeat (or at the very least remove) all enemies, then she Concentrate (DC 20, with the DC going up if she is wounded) to change back.
I'm also wondering if it is possible to manifest Selene's Devices (Sword and Coat) to appear with the change and disappear when not in use, along with any power effects Selene has active — namely her aura, which is innate to her, continous and permanent.