It is easy to concatenate the values of two columns in a new column. However can I do this in my code without touching the sheet.

Argument tree is the first_col and it should be the concatenation of two columns (1 + 4)

It should be a number of a virtual column.

Call Get_Duplicates(duplicates, 1, 1, last_row)

Sub Get_Duplicates(ByRef duplicates() As String, first_row As Long, first_col As Long, last_row As Long)