Option Explicit

Sub Test()
    Const LoBound As Long = 1
    Const UpBound As Long = 25

    Dim vA As Variant
    Dim vB As Variant
    Dim avC(LoBound To UpBound, 1 To 1) As Variant

    vA = Range(Cells(LoBound, 1), Cells(UpBound, 1)).Value
    vB = Range(Cells(LoBound, 2), Cells(UpBound, 2)).Value

    Dim i As Long
    For i = LoBound To UpBound
        avC(i, 1) = vA(i, 1) & vB(i, 1)
    Next

    Range(Cells(LoBound, 3), Cells(UpBound, 3)).Value = avC
End Sub