Userform ComboBox dropdown list from multiple ranges

Question

I have a userform where a ComboBox should already be populated with values from three different excel worksheets. I have no trouble creating a dropdown list using the .List property of the combobox when it is one range, but this has me completely stumped. A few of my various attempts below:

Private Sub Userform_Initialize 
Dim A as Variant
Dim B as Variant
A = Worksheets("SheetA").Range("A:A")
B = Worksheets("SheetB").Range("A:A")
With ComboBox1
ComboBox1.List = A + B
End With
End Sub

This generates a type mismatch error

ComboBox1.List = Array(A, B)

This generates an empty dropdown list

I have also tried defining A and B as ranges to try to use Union on them, but it seems they need to be variants for the

A = Worksheets("SheetA").Range("A:A")
B = Worksheets("SheetB").Range("A:A")

lines to work

Answer 1

This code will list the columns A in SheetA and SheetB in ComboBox1

Sample data as shown below;

Column "A" in SheetA	Column "A" in SheetB
a1	b1
a2	b2
a3	b3
a4	b4
a5	b5
a6	b6
a7	b7
a8	b8
a9	b9
a10	b10
a11	b11
a12	b12
a13	b13
a14
a15
a16
a17

.

Private Sub UserForm_Initialize()
    Dim arr1, arr2, arr3
    
    arr1 = Application.Transpose(Sheets("SheetA").Range("A1:A" & Sheets("SheetA").Range("A" & Rows.Count).End(xlUp).Row))
    arr2 = Application.Transpose(Sheets("SheetB").Range("A1:A" & Sheets("SheetB").Range("A" & Rows.Count).End(xlUp).Row))
    
    arr3 = Split(Join(arr1, ",") & "," & Join(arr2, ","), ",")
    
    ComboBox1.List = arr3
End Sub

.