The way around this problem is to create a single INDEX into the SuperFile, as shown by this code (contained in IndexSuperFile4.ECL):

IMPORT $;

F1  := FETCH($.DeclareData.sf1,
             $.DeclareData.sk2(personid=$.DeclareData.ds1[1].personid),
             RIGHT.RecPos);
F2  := FETCH($.DeclareData.sf1,
             $.DeclareData.sk2(personid=$.DeclareData.ds2[1].personid),
             RIGHT.RecPos);
Get := PARALLEL(OUTPUT(F1),OUTPUT(F2));

SEQUENTIAL(BUILDINDEX($.DeclareData.sk2,OVERWRITE),Get);

When you use a single INDEX instead of a SuperKey, the FETCH operations once again retrieve the correct records.