export data to multi-sheet .xls in unix

For example i am having some sql scripts running and producing some input:
…
sqlplus -SILENT $user/$dbpass@$host @group_report.sql1 >/tmp/file1.xls
sqlplus -SILENT $user/$dbpass@$host @group_report.sql2 >/tmp/file2.xls
..
is there any util for to merge file1 and fil2 into multi-sheet .xls? if not is there any chance to get an Excel api without using java libraries.. might be php?