If the first line segment in the callout named "co1" has a fixed length, this example specifies that the length of the first line segment in the callout named "co2" will also be fixed at that length. For the example to work, both callouts must have multiple-segment lines.
With ActiveDocument.Shapes
With .Item("co1").Callout
If Not .AutoLength Then len1 = .Length
End With
If len1 Then .Item("co2").Callout.CustomLength len1
End With