You could use a delimiter to put both values together and then delimit them once you pull them out.
ddList.DataValueField = obj.ID + "~" + obj.TestID;
Otherwise you could create a dictionary that hold the ID and the Test ID, so that when you select your item from the dropdown, you can use the value field as the key to the dictionary and select the TestID.
private Dictionary<string, string> referenceValues = new Dictionary<string, string>();
referenceValues.Add("ID", "TestID");
ddList.DataValueField = "ID";
//When pulling then value back
var referencedItem = referenceValues[ddList.selectedValue];