I have just finished building a Robot with a similar problem.
Assuming DT1 and DT2 are as you specified, I would use the Programming.DataTable.JoinDataTables activity to join DT1 and DT2 using a Join Type = FULL (aka Outer Join in SQL), Join on DT1.id = DT2.id and output DT3.
DT3 will have columns id, amount and amount_1 (assuming the amount column is named the same in both DT1 and DT2, the join activity automatically makes the second identical column name unique by adding _1).
Then use Programming.DataTable.AddColumn to add the “Total Amount” column to DT3.
If you want to rename the Amount and Amount_1 columns, you can do so by Invoking a little VB.Net code"
DT3.Columns(“Amount_1”).ColumnName = “My New Name” . (Or something close to that, I have not tested it).
And then finally a simple for each row activity setting Total = Amount + Amount_1. You will have to handle the possibility that amount or amount_1 can be nothing as they will not default to 0 when the join does not match id’s in both tables. Do an if test on amount_1 is nothing then amount_1 = 0 before the addition assignment.