Split View is a new view option available in Visual Studio 2008 in addition to the existing “Design View” and “Source View”. With Split View option selected, you can have your HTML View and Design View side by side. This would make it a lot easier for you to see how your HTML changes impact the design view and vice versa.
This feature is available both in Visual Studio 2008 and in Visual Web Developer 2008. By default VS enables split view horizontally but you can make the split view work in vertical mode as well by doing the following settings:
Go to Tools Options –> HTML Designer settings –> Check Split views vertically-> Click OK
Split view will now show the two windows vertically as seen below.
SDET, Web Development Tools