I have come across a few discussions on the topic of using Visual Studio 2012 along side Visual Studio 2010 while targeting .NET 4.0 from both of these IDEs. While in principle there shouldn’t be a problem adopting this strategy, it is important to be aware of some practical issues that exist in the above scenario.
Basically .NET 4.5 is an in-place upgrade of .NET 4.0 and replaces 4.0. It is not a side by side install (like 1..1, 2.0 etc. were). These two don’t co-exist on a box. There are quite a few problems that come up while using VS 2012 for applications that target .NET 4.0 runtime. Depending upon the client OS and the framework selection you make in Visual Studio an application can display selective bugs to a few client desktops whereas running fine for the others. This leads to further troubles as if you are using VS 2012 to build applications that target 4.0, you will not see these bugs at the dev time and hence will not be able to patch/fix them. What makes it further troublesome is that some of the most prominent problems appear on WPF space and while the stress these days is on Web based apps a lot of traditional organisations such as investment banks have a very heavy thick client application population.
These links will highlight the issues I refer to
So my advice to all development teams to be aware of the nature of applications that they develop and their audience. If for example your applications predominantly consist of web based apps, redistributable components, WWF or WCF applications then you should be fine using Visual Studio 2012 to target .NET 4.0. Examine your individual development scenarios and then take a decision on whether to upgrade to VS 2012 for .NET 4.0 development or not.