Adding existing forms or components with additional *.Designer.pas files doesn’t work completly.
The files are copied and added to the project, but you have to manually edit the *.elements file if you want to invoke a designer.
This:
<Compile Include="Form2.pas" />
<Compile Include="Form2.Designer.pas" />
must be changed to this:
<Compile Include="Form2.pas">
<SubType>Form</SubType>
<DesignableClassName>WindowsApplication1.Form2</DesignableClassName>
</Compile>
<Compile Include="Form2.Designer.pas">
<SubType>Form</SubType>
<DesignableClassName>WindowsApplication1.Form2</DesignableClassName>
</Compile>
Applies to SubType “Component” as well.