If you are using Visual studio 2015 you will see Visual studio have option for new window for .CS file but not for .Cshtml file. Didn't this thing look strange to you.
Here is how to fix it.
HKEY_CURRENT_USER\SOFTWARE\Microsoft\VisualStudio\14.0_Config\Languages\Language Services\HTMLX\Single Code Window Only
Go to Regedit and find this location in left side. Set the value of "Single Code Window Only" to 0 and it will work.
In my case it's revert to 1 after few days. I didn't get it how but you can bookmark this so in future you don't need to spent time on browsing.
Happy Coding !