if Fen'harel absorbet Mythal then it would mean that one person has the power from two gods.
the question is ..was it a absorb of the spirit of mythal or only of the power of mythal..or both?
The dev notes say that Flemeth is sending her god soul thru the eluvian to Morrigan before accepting her fate into Solas hands. So what Solas absorbs is the power, not the spirit. I think?... But then again, even with the dev notes, its hard to say how all of this will play out. Flemeth is a powerful witch even without Mythal, so it could be that she sent a spark of Mythal to Morrigan, but left most of the power in herself so that Solas could extract it and use it for whatever he intends to do? She can probably fragment her soul into pieces (the amulet she gave Hawke most likely held a fragment of her soul), so maybe she split it again, giving a part of it to Morrigan (which would give her back her free will, since she would be in possession of the soul which she is bound to) but still retaining enough in her to give Solas the power he needs.
Meh im scratching my head too much, gonna make a hole in it.... Lets just wait. Either DLC or DA4 will give us some answers. Presumably, Flemeth is dead. But i dont think this was the last we saw of her. The Flemeth as we know it probably wont show up again, but there's so many ways of Flemythal to resurface again that i dont think we've seen the last of this entity. Dont forget "She was betrayed, as I was betrayed, as the whole world was betrayed". Flemythal is looking for vengeance, something she's been planning ever since Mythal fled as a wisp to merge with Flemeth(Andraste first maybe?). So either this vengeance fits into Solas plans, or there is no way in hell she would give that plot up, she sounded too hurt and upset in the fade to have such a change of heart from that scene until the last scene of the game. She will have her vengeance. "Alas, so long as the music plays, we dance". The music (most likely refering to the Calling") is still playing and is still a great mistery. So she will keep dancing until all misteries of Thedas are resolved.
On a side note: This is actually pretty straightforward in my playthroughs, cause all my world states have Morrigan doing the dark ritual. In that case, and considering the dev notes, what i see from that last scene is Flemeth sending Mythals soul to Morrigan and giving Urthemiel's soul (previously removed from Kieran) to Solas. But then again i cant think of it as cannon because there are lots of people who dont even have Kieran in the game, meaning that last scene (which plays out the same in both situations) could have a completely different meaning.