Although it’s recommended that we install Visual Studio versions in the order in which they were released (for example, install Visual Studio 2012 before we install Visual Studio 2013) as mentioned in http://msdn.microsoft.com/en-us/library/ms246609.aspx, there is possibility that we have installed Visual Studio 2013 before we know that we need to install Visual Studio 2012.
It happened to me last month. I actually more than happy to have Visual Studio 2013 only in my computer as I can also use it to open Visual Studio 2012 projects. But in the end I need to install Visual Studio 2012 because one of my teams are using Visual Studio 2012 and I don’t want any incompatibilities or project upgrades affect them.
If you have to install Visual Studio 2012 after Visual Studio 2013 for any reason, don’t worry, it can be installed without any issue.
I have been using this configuration almost a month and there is no issue at all. I mostly use them to do ASP.NET projects (and related, such as class library, etc) and I’m sure there is no issue with other project types too.