Splitting data out over a predefined percentage and using that data for new calculations

KatrienVds 
No need to select all columns. We will follow your strategy. Use this code to generate the appended header table.

Order Header 2 = 
VAR Owner1Table =
    SELECTCOLUMNS ( 
        'Splitted orders 1', 
        "@Order No", 'Splitted orders 1'[Order No], 
        "@Owner", 'Splitted orders 1'[Owner 1], 
        "@Percentage", 'Splitted orders 1'[Percentage] 
    )
VAR Owner2Table =
    SELECTCOLUMNS ( 
        'Splitted orders 1', 
        "@Order No", 'Splitted orders 1'[Order No], 
        "@Owner", 'Splitted orders 1'[Owner 2], 
        "@Percentage", 1 - 'Splitted orders 1'[Percentage] 
    )
VAR SplittedOrders =
    UNION ( Owner1Table, Owner2Table )
VAR OrderHeader = 
    SELECTCOLUMNS ( 'Order Header', "Order No", 'Order Header'[Order No], "Owner", 'Order Header'[Owner] )
VAR NotSplittedOrders =
    ADDCOLUMNS ( FILTER ( OrderHeader, NOT ( [Order No] IN VALUES ( 'Splitted orders 1'[Order No] ) ) ), "Percentage", 1 )
VAR Result =
    UNION ( NotSplittedOrders, SplittedOrders )
RETURN
    Result

Then make tthis table in between the two tables exactly as you have suggested