create a union table with new column names

You need to rename the column in the 2nd table River . You can do it before Append operation, or during the append operation. For example, here is Table1 and Table2

THis code does a Union into the new SPMO column

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMlSK1YlWMgKTxkqxsQA=", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [SalesOrder = _t]),
    #"Appended Query" = 
        Table.Combine(
            {
                Table.RenameColumns(Source, {{"SalesOrder", "SPMO"}}),
                Table.RenameColumns(Table2, {{"PurchaseOrder","SPMO"}})
            }
        )
in
    #"Appended Query"

How to use M code provided in a blank query:
1) In Power Query, select New Source, then Blank Query
2) On the Home ribbon, select "Advanced Editor" button
3) Remove everything you see, then paste the M code I've given you in that box.
4) Press Done
5) See this article if you need help using this M code in your model.

 

Here is my PBIX file to help you see it work.