When you try to add multiple existing projects to one solution, you will find it will failed at the second adding because after you added the first project, the solution disappeared, so you were not be able to add one more. The following is the solution.
Step 1: Select Tools menu in Visual Studio.
Step 2: Click on Options…
Step 3: Check “Show All Settings” at the bottom of popup-window
Step 4: Select “Projects and Solutions” from items available on left pane.
Step 5: Check “Always show solution” from right pane as shown in figure.