//Assume you have done with the update table
//From the second query, put extra where to it, i.e
$sql = "SELECT SEQ FROM TABLE WHERE FIELD='NEXTINV'";
$query = $this->db->query($sql);
$row = $query->row_array();
$seq = $row['seq']; // here it is
If I'm understanding your question correctly, no, you wouldn't be able to combine the two since MySQL's UPDATE statement only ever returns the number of rows that were actually changed and not the resulting value.
I haven't tried this myself but perhaps you can wrap these statements in a stored procedure in MySQL so at least both statements run on the database instead of making two separate PHP calls but if the goal of your question is to eke out more performance, there are probably better ways of doing that.