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