If table schema is same you can use Marge method in your code. than add table to your data-set.
DataTable ONdt = new DataTable();
DataTable OFFdt = new DataTable();
string strsql = "SELECT * FROM **serverA_table**";
ONda = new SqlDataAdapter(strsql, ONcon);
ONds = new DataSet();
ONda.Fill(ONdt);
strsql = "SELECT * FROM serverB_table";
OFFda = new SqlDataAdapter(strsql, OFFcon);
OFFds = new DataSet();
OFFda.Fill(OFFdt);
OFFdt.Merge(ONdt);
OFFds.Tables.Add(OFFdt);