Concatenating two lists of numbers of different lengths

If you prefer VBA:

Sub Expand()
    Dim r As Long
    Dim m As Long
    Dim s As Long
    Dim n As Long
    Dim t As Long
    Dim u
    Dim w
    Application.ScreenUpdating = False
    m = Range("U1").End(xlDown).Row
    u = Range("U1:U" & m).Value
    n = Range("W2").End(xlDown).Row
    w = Range("W2:W" & n).Value
    Range("O2").Resize(m * (n - 1)).NumberFormat = "@"
    t = 1
    For r = 1 To m
        For s = 1 To n - 1
            t = t + 1
            Range("O" & t).Value = u(r, 1) & w(s, 1)
        Next s
    Next r
    Application.ScreenUpdating = True
End Sub