God, I'm slow getting to this. Anyway...

Overall, it's pretty well done. I'll focus primarily on things to improve on. First and the main thing is the musculature. The details done are good and fairly accurate, but a few of them seem a bit off. Mostly on Nanashi rather than Tabias, I believe the bicep should flow down to the forearm a bit more smoothly. It feels a bit thin near the elbow.

His right pectoral, as well, seems like it should both go down a bit further on his torso and bulge outward a slight bit more considering the detailing on the rest of his body.

Nanashi's right shoulder also seems a bit undersized, and there doesn't appear to be enough foreshortening with the rest of his body to justify it.

For a change of pace (and to stop picking on Nanashi for a moment), Tabias seems to have much better proportions. The main things that could use improvement with him are his left eye (seems small) and his thumb caressing the hair should be a bit larger. Other than that, he looks quite well done.

Back to Nanashi, there are two other main things about him that seem off. First, his neck seems too thin closer to his head, though his hair does a good job of hiding it. Last, the symbol below his left eye should follow the curve of his face - it seems like it was done on a flat surface, and the humanoid face doesn't really do that.

Other than those things, I can't really find many ways you could improve on this image. Solid work like usual, Marina, I can definitely feel the...erm, 'assion', between the two.

Sorry if this critique comes off as negative - when I offer them, I rarely look at what's right, only what needs/could use improving.

I like the ciriq-thingy! We subscribers deserve more features!
I watched your video and it was interesting to see how you work with coloring.

Hmm... I think I'll start with the Anatomy:

Good:
The hand holding the hair is really good, I see it as an improvement.
The overall anathomy on the bodies are nice. May I ask you... did you use my MS Paint video as a reference for the guys muscles? xD

Could be improved:
I have discovered that you usually makes pretty small thumbs compared to the rest of the fingers. I suggest you try out to make the thumbs a little bit longer next time you draw a hand. Also, the other hand look pretty stiff. Don't forget the joints in the fingers, a finger is "split" in three parts. And every part is bent a little bit when the hand is relaxed.
tip: take a look at photos of skinny and boney hands to study the bone structure of the hand.

The shoulders on the dark-haired guy have different sizes. Have seen this on other images you've done too and I think you could train more on the male anathomy.
Okay enough with the anatomy xD

Coloring/shading:
What I like:
Different skintones is a nice touch, it separates the characters well. And I think you did right by using warm colors also in the shading.
What could be improved:
What bothers me is lack of light source, I'm unsure if it's from above or from the left. But I guess it's from above.
What bothers me the most here is that you give everything a small shadow. Both characters have pretty long hair and bangs there should be more shadows in their faces. And keep in mind things like the chin/necks impact on the shoulders.
Both should have a larger shadow on their necks.
Also don't forget the cloths, they cast shadows on the bodies aswell.
PROTIP: when you shade muscles, especially men's torsos, avoid doing so much lineart and let the shadows do the job. This way the muscles will turn out more natural for the human eye. Especially on soft-shaded images like this one.

Other:
I really like the background, I love the pattern you used and fits well with the motive.

okay I think I'm out of words now...xD