Hi Sandip,
Thank you for evaluating Syncfusion Products.
You can derive LineTool, BezierCurveTool and LineBaseTool class and you can draw the double line in the ProcessMouseUp event of LineBaseTool class. Please refer below sample which illustrates this:
http://help.syncfusion.com/support/Diagram.Windows/F93854_Diagram/main.htmIn this sample, you can set OffsetX / OffsetY values of the Line and BezierCurve by using the comboboxes. You can activate the customized LineTool and BezierCurveTool by using Tool->LineTool and Tool->BezierTool menuitems.
Please let me know if this helps.
Thanks,
Melba