Hi Stefan ,
You can assign the popupControlContainer to the PopupControl property of comboDropDown to show the Popup. Please refer to the following code snippet.
this.comboDropDown1.PopupControl = this.popupControlContainer1;
this.comboDropDown2.PopupControl = this.popupControlContainer2;
Please refer to the attached sample that demonstrates this completely.
http://websamples.syncfusion.com/samples/Tools.Windows/F70391/main.htmLet me know if you have any other questions.
Thanks,
Nagaraj