using different database (SID) in a subselect with SQL*Plus / Shell Scripts

I have this simplified SQL statement within Shell Script.

function ()
{
sqlplus -S $DBO/$DBOPW_at_$ORACLE_SID <<EOF > /opt/test/output.$(/bin/date '+%d%m%Y.%Hh') set echo Off
[...]
set linesize 20000

SELECT testfield from test where testfield not in (select testfield from test2); exit
EOF
}

The problem is that the subselect (select testfield from test2) is an other oracle database (not the one connected on with sqlplus). Howto to that with SQL*Plus and Shell Scripts?

cheers Sven

--
http://www.freelists.org/webpage/oracle-l