If you have two separate solutions, then you cannot view them simultaneously in the same VS instance. However, you can open multiple instances of VS and tab between them.
An alternative would be to import the projects from one solution into the other, thus putting all of your projects into one solution. You can do this by following these steps:
In the Solution Explorer, select the Solution into which you want to import a project. Right-click, and select Add->Existing Project from the context menu.
In the file chooser, find the project file that you want to import (it will end in .XXproj where XX is the language you're using).
Click Open and voila! Your project is imported.