Question: I am an editor for a trade publication with 40 years of experience in journalism. The company wants me to take a job on the events side, writing content, helping with organization, moderating panels, archiving video. They haven’t called it a promotion but it would clearly be more intense work and involve traveling to conferences every other month. I’d like to accept the offer but I want to negotiate a salary increase after a six-month review. Can I do that upfront or do I wait the six months and in either case, what kind of percentage would be equitable?

Answer: 
You are playing well above your pay grade. Let your manager manage your colleague. An outcome like "the project got finished but wasn't great, one of the interns worked hard but the other was a nightmare" is common as dirt in this industry. 

Do the tasks your manager has assigned to you. When you're out of tasks, go ask for more. When you can't do something because you're waiting on the colleague, let your manager know. When you get something from your colleague and it's unusably bad, ask your manager whether you should fix it up, or bounce it back explaining why you can't use it. Then ask what you should do next given that you can't do what you were planning to do (use the thing from the colleague.)

You seem to think that if you can't fix this other intern (or arrange for a firing) it will reflect poorly on you. Do you really think your manager is so terrible as not to see what you see? You've reported it, shown the code, the manager has already started dealing out tasks and telling the other intern to use source control, and so on. This is all the manager's job, not yours. Relax and do yours, keep your manager informed, and let the manager manage.
As other recommend, I would ask somebody higher up if I could start CCing him in all your exchanges besides documenting everything in a source code system. 

Obviously, if you working for the two, your colleague does not need to do it. However, it seems a bit odd he taking such a relaxed stance of missing so many hours of work and days off in the middle of an intern project...it is as he knows he cannot fail.

I would investigate if he has relatives inside. I have seen my share of pet employees over the years. It is usually not that hard to find out. Maybe you were setup together for a reason.

Welcome to the fantastic world of the real world and office politics!

Assuming no foul play, at the end of the day, someone is failing you not making regular meetings to guide you both and assess the state of the project.

PS. A possible course of action if that is a University endorsed internship is probing on the University side if you can change for another place. The odds are slim, but without asking it is not possible to find out.