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Nice formula Bernd; however, I would suggest this modification to it so that it can be copied down into blank rows (in anticipation of future entries)...
=IF(AND(B2="",C2=""),"",LOOKUP(2,1/(1=FIND(LEFT(" "&B2,ROW(INDIRECT("1:"&1+LEN(B2))))," "&C2)),ROW(INDIRECT("1:"&1+LEN(B2)))))
I would note for the OP that this formula is entered normally (that is, it is NOT an array-entered formula). And I would also note that the formula is is case-sensitive; the case-insensitive version of it would be this...
=IF(AND(B14="",C14=""),"",LOOKUP(2,1/(1=FIND(LEFT(" "&UPPER(B14),ROW(INDIRECT("1:"&1+LEN(B14))))," "&UPPER(C14))),ROW(INDIRECT("1:"&1+LEN(B14)))))