Concate two sessions in to one session

Error:
Unable to cast object of type 'System.String' to type 'System.Data.DataSet'.

first you have to get value in two tables/dataset and then you can merge it into one table/dataset.

how can i bind this:

ds = SqlHelper.ExecuteDataset(CommonSetting.GetConnectionString(true), CommandType.StoredProcedure, strSql, param5, param6);

if (Session["Table"] != null)

{

string s1 = (Session["Prac"].ToString());

Session["Table"] += s1;

}

else

{

Session["Table"] = Session["Prac"];

}

DataSet ds1;

ds1= (DataSet)Session["Table"];
dldisplayMessage.DataSource = ds1;
dldisplayMessage.DataBind();

please explain why and which type do you want to concatinate ?