Each VS solution can have multiple projects, so... you can just add each Application as a Project in your solution.
One way to do it would be:
- Create a new BLANK/EMPTY solution
- Right click on the solution name and select "Add > Existing Website" and select your ROOT folder.
- Repeat step 2 but this time select the folder "Application 2"... do the same for all projects.
At the end you will end up with something like:
- SOLUTION
+- Root
+- Application 1
+- Application 2
Hope it helps.