From the Editor, adding a new code to the project launches a new instance of Visual Studio when asked to view it, instead of putting focus to the instance allready open.
Are you running Visual Studio Express? DTE support doesn’t work with express - so we can’t correctly load it in the existing instance 
Cheers,
Nick (Epic Games)
No, VS 2013 Ultimate
How are you adding the new code that’s causing it? I’ll take a look if you can give me a repro case.
Just from the Editor. File->Add new code to project, then from the dialog selecting yes on the question if desire to edit code now.
I found there was a lingering Visual Studio 2008 C++ Express on my pc. After I uninstalled that, the next time I tried it, the second instance was not opened of 2013. So, I guess the error was because there was a version of Express installed and it was messing things up.
Cool 
Though I’m a little surprised having 2008 express installed caused it.
I had the hardest time being able to generate the Visual Studio files because of how many different versions of Visual Studio I had installed, I missed one in uninstalling them. It’s working for me now, that I only have one version of VS installed. 2013 Ulitmate.
Actually, it seems this behavior occurs when the Editor is run from Visual Studio, and then add code files.
Ok - this is a real bug. It looks like when you add the code it cause the project to be regenerated. Visual Studio launches that dialog asking if you want to reload the project. If that dialog is open it horks the DTE code-path and we end up starting another visual studio, likely because the existing process is unable to respond to the DTE request while in the modal dialog loop.
I’ll take a closer look tomorrow morning, but the way around it for now, is try tabbing over to VS and saying ignore or reload, before saying [YES] when it asks to open it.
Cheers,
Nick (Epic Games)
Thanks for looking into this, especially at such a late hour.
Hi ,
I’ve fixed the bug that was causing this,
we now batch up multiple open file requests and open them all at once after verifying the right instance of visual studio so we don’t get hosed by the reload dialog that pops up when focus shifts after opening the first one!.
It should be fixed in the next official release (about a month from now). If you’re compiling from our bleeding edge source from Github, you should see the fix the next time we do a push (possibly within a few days).
Cheers,
Nick (Epic Games)
Nice, Thank you
Has there been any update on this bug?
This was fixed in 2014, see below. If it’s happening again to you I’d open a bug. For me I’m not running into this issue any more, it works correctly.