Hello,

Step 3 is not valid. Designing the form must happen only by double-clicking the main form class RadRibbonForm1.cs. 

To illustrate that this is not a Telerik-related issue try the same steps with an MS Form. Add Form1.Test.cs file and add some controls to the partial Form1.Test.cs opened at design time -> a compile-time error is shown: 
Error CS0111 Type 'Form1' already defines a member called 'InitializeComponent' with the same parameter types.
Designing a form from a different partial class is not valid action!

The Windows Forms Designer does not support dividing a designable type's definition into more than two partial implementations. This restriction includes creating a new class file that contains a third partial definition of the type, as well as adding a third partial class definition of the type in either the main file or the designer file. 

Here are some resources for similar problems:
https://stackoverflow.com/questions/21822980/a-partial-class-has-multiple-formhttps://social.msdn.microsoft.com/Forums/ie/en-US/79910eaa-84e7-472d-95ee-4b8ddfbf99f0/multiple-partial-class-files-cause-problems-with-forms-designer?forum=winformshttps://stackoverflow.com/questions/37923301/add-partial-form-class-below-windows-form-nodehttps://stackoverflow.com/questions/2485851/winform-partial-classes

Regards,
Dimitar
Progress Telerik

Hello, Bastian,

The possible solution that I can suggest is to use a RadForm and add a RadRibbonBar from the toolbox instead of using a RadRibbonForm. The attached gif file illustrates who to do it.

I hope this information helps. If you need any further assistance please don't hesitate to contact me. 

Regards,
Dess | Tech Support Engineer, Sr.
Progress Telerik