No. You decide first on which twin you want to play and stick with it. I started with Lumine and lucky for me there is enough fan art for both. LOL
You can only go back by choosing Aether on a new account.
Many want to switch since Aether is “canon”, but I think it is very nice that they integrate both sides. My friend has chosen Aether and we can always compare e.g. Dainsleaf’s words when you meet him or other lore where they switched the gender to dig deeper into everything. 😀