Combining 2 tables both have duplicate values, need help!

And here is another method that seems to handle entries that are in one table but not the other, with or without data for the respective entries:

let
    CostTbl = Table.Group(Cost, {"Item"}, {
        {"Costs", each Table.AddIndexColumn(_,"Index",0,1),
        type table [Item=nullable text, Cost=Int64.Type, Index=Int64.Type]}}),
    #"Rename Cost Item" = Table.RenameColumns(CostTbl,{"Item","CostItem"}),
    OrderTbl = Table.Group(Order, {"Item"}, {
        {"Orders", each Table.AddIndexColumn(_,"Index",0), 
        type table [Item=nullable text, Units=Int64.Type, Index=Int64.Type]}}),
   
    Join = Table.Join(#"Rename Cost Item","CostItem",OrderTbl,"Item",JoinKind.FullOuter), 
    #"Removed Columns" = Table.RemoveColumns(Join,{"CostItem", "Item"}),

//blank table used in next step
x=#table({"Units","Orders","Cost"},{{null,null,null}}),

    #"Added Custom" = Table.AddColumn(#"Removed Columns", "Cost+Orders", 
        each Table.Combine({[Costs]??x,[Orders]??x}), type table),
    #"Removed Columns1" = Table.RemoveColumns(#"Added Custom",{"Costs", "Orders"}),
    
    #"Group Tables" = Table.TransformColumns(#"Removed Columns1",{"Cost+Orders", 
            each Table.Group(_,"Index",{
                {"Item", (t)=>List.RemoveNulls(t[Item]){0}?},
                {"Units", (t)=>List.RemoveNulls(t[Units]){0}?},
                {"Cost", (t)=>List.RemoveNulls(t[Cost]){0}?}               
                }), type table[Item=text,Cost=Int64.Type,Units=Int64.Type]}),

    #"Expanded Cost+Orders" = Table.ExpandTableColumn(#"Group Tables", "Cost+Orders", {"Item", "Units", "Cost"}),
    #"Removed Blank Rows" = 
        Table.SelectRows(#"Expanded Cost+Orders", each 
            not List.IsEmpty(List.RemoveMatchingItems(Record.FieldValues(_), {"", null})))
in
    #"Removed Blank Rows"