I assumed a semi-colon as seperator, and B and C as the other shapes


Option Explicit
'   A to B and C
'   alt text in A = "This is alt text ; This will appear in different shape"
'   "This is alt text" to B
'   "This will appear in different shape" to C

Sub changeText(oShape As Shape)
    Dim s As String
    Dim i As Long
    
    s = oShape.AlternativeText
    i = InStr(s, ";")
        
    oShape.Parent.Shapes("B").TextFrame.TextRange.Text = Trim(Left(s, i - 1))
    oShape.Parent.Shapes("C").TextFrame.TextRange.Text = Trim(Right(s, Len(s) - i))
End Sub