Word doesn't have a practical way of inserting just the first 2 letters of the names into the header unless there's a separate data field for those letters. If, however, you were to apply unique character or paragraph Styles to the ranges containing the name mergefields, you could use StyleRef fields in the header (and/or footer) to replicate the complete first and last names.

Even the impractical way would require the above approach, but then it would need to be coupled with 52 IF fields for the first names and another 52 IF fields for the last names to extract the first & 2nd letters from each.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]