You've done an excellent job with your color choices here. The choice of green in your background really works to set them in an other-worldly place. The brush you used for it works, but I think with the way the characters are kind of flowing, having a bit more movement to your background would really compliment the action in the foreground.
Where I believe this falls short is in defining and separating the characters. I believe they're a little too similar in color to easily identify where one ends and the other begins. A quick adjustment to one of them to maybe add a little more blue would really help.
Your rim light works and helps to define the edges, but the interior portions of their forms need to be defined a little better. It took me a while to notice that Venom's thigh was outside of his arm and not the other way around. Some stronger highlights and a defining rim light over the top of his thigh where the light would be hitting it would really help.
You did and excellent job in lighting Spawn's cape here. Notice how you've got it brighter at the top where the lightsource is strongest? The same thing needs to happen for everything in the picture. The black areas of Spawn should appear brighter towards the top as he's closer to the lightsource. The same thing with Venom. The way you have it rendered now is very even and that's helping to contribute to the problem of not being able to define the forms. A little variation and some stronger highlights here and there will be a great difference maker.
Lastly, keep and eye on how dark you're making things. When it gets to the point where the lineart is difficult to see, you've gone too dark. That will be come a real issue when you work for something in print. Best to try to keep your ink limits under 240 as much as you can. This will keep everything easily readable.
Overall, this is very good work. A few tweaks here and there and this can turn into something really special. Keep it up!