switch from one project to another in one solution

Selecting the project you are working on is pretty easy: right click the project name in the Solution Explorer pane, and select the "Set as StartUp Project" option. When you run the solution, this is the project that is executed.

Accessing a form from a different project in the same solution is easy: just add a reference to the project:
Right click the project that doesn't contain the form.
Select "Add Reference..."
In the resulting dialog, select the "Project" tab on the left hand side.
Highlight the project containing the form, and press the "Add" button.

You may need to add an Imports statement to your files before you can access the form.