Artist didn't specify who they are, so I tagged them as Alice and Sakuya (becaues they look like those two, durr).
Google also returned that those two are probably Alice and Sakuya, but you be the judge because I'll leave those tags alone.

Hrm...Alice has short hair, but her eyes are blue. I was going to say Flan, but her hair is long enough to sport a side-ponytail, plus her eyes are red. Marisa's hair is on the long side, too, but the eye colour appears to match. Hrm, talk about confusing. No arguments about Sakuya, though. Aside from the lack of twin braids, the appearance matches. I wonder if this is a MarixSaku picture with alternative hair styles?

I'll leave the tags be for now. Until (or if) the artist adds the tags stating who they are, we can only speculate for the time being.

For some reason I thought it was Youmu and Shou, although it's hard to tell without their identifying garb and Myon.

Alice is depicted with different eye colors with yellow being one of them, so it's probably her.

Resembles a bit too much post #1428809 and post #1439000 to call it a coincidence. Which is understandable, since the artist described this one as a practise work.
Therefore tagged this as derivative_work.

Alice is depicted with yellow eyes sometimes, but Marisa is also depicted with shorter hair sometimes as well and she has yellow eyes.

So it's a toss up overall

I say it's Merlin and Lunasa.

Artist is named the same as the fictional artist from Sakurasou who inspired me to take up drawing. Apparently a very inspiring character.