Guity,
Not very sure if this is what is required... but below are my 2 cents...
=IF(ISNA(INDEX($B$8:$D$13,MATCH(1,($B$8:$B$13=Sheet1!$B8)*(Sheet2!$C$8:$C$13=Sheet1!$C8),0),3)),Sheet1!$B8&Sheet1!$C8,INDEX($B$8:$D$13,MATCH(1,($B$8:$B$13=Sheet1!$B8)*(Sheet2!$C$8:$C$13=Sheet1!$C8),0),3))...