Koha doesn't have support for merging records yet.

http://old.nabble.com/Merge-Bib-Records-tt18611514.html

But there is a way to do it:

• use SQL to move items by changing items.biblionumber and items.biblioitemnumber to point to merged bib
• run misc/maintenance/sync_items_in_marc_bib.pl to update item information embedded as tags in the bib record
• reindex