You'd better make proper use of VBA.
1. Never use Additem to populate a ListBox or Combobox
2. Use the propertuy 'List' to populate a Combobox/ListBox
3. Use the Initialize Event in a Userform to populate Listboxes/comboboxes
In this Case:
Private Sub UserForm_Initialize()
For Each it In ThisWorkbook.Sheets
If Left(it.Name, 2) = "PE" Then
c00 = c00 & "_" & it.Cells(3, 4)
c01 = c01 & "_" & it.Cells(10, 4)
End If
Next
ListBox1.List = Split(Mid(c00, 2), "_")
ListBox2.List = Split(Mid(c01, 2), "_")
End Sub
See: https://www.snb-vba.eu/VBA_Fill_comb...istbox_en.html