The datatable1.Merge(datatable2) statement will merge datatable2 into datatable1. You cannot retrieve the old values in the datatable1 after the merge happens.
A quick way to do that, is by creating a copy of the first datatable and then doing the merge in the following way.
Dim dtMerged As DataTable = New DataTable()
dtMergerd = datatable1.Copy()
dtMerged.Merge(datatable2, True)
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