Maybe this problem has been resolved in 4.5

But

with IBOQuery do
begin
close;
SQL.Clear;
SQL.Add(Select * from TableOne);
SQL.Prepare;
open;

close;
SQL.Clear;
SQL.Add(Select * from TableTwo);
SQL.Prepare;
open;

If TableTwo has different Field\Names than TableOne, I get errors on
FieldNames not found from TableOne. Tried everything to force some kind of
clear internal FieldNames.
But I did get it to work by inserting a dummy ExecSQL like. The IBOQuery is
never populated via a right click -> add all fields

with IBOQuery do
begin
close;
SQL.Clear;
SQL.Add(Select * from TableOne);
SQL.Prepare;
open;

close;
SQL.Clear
SQL.Add('Commit');
ExecSQL;

close
SQL.Clear;
SQL.Add(Select * from TableTwo);
SQL.Prepare;
open;

With the addinal ExecSQL, the internal fieldnames obtained from TableOne
seem to be cleared at the time TableTwo is queried and no error occur.