I think I can help with that one.
IDX1 would have the 3 columns with the data ordered by OrderID, then ItemID, and finally Status. If there were more columns in the table, you would need to use a key lookup to look at the row on the actual table.
IDX2 on the other hand will still have 3 columns, but is ordered by OrderID, then ItemID and Status is not part of the ordering. Status is just included in the index so if your select included the Status, you would not need to use a key lookup to find the status in the actual table. Any columns that you select outside of OrderID, ItemID, and Status will still need a key lookup back to the main table.
Since IDX2 does not have any order on the Status, if an OrderID + ItemID can produce multiple Status's (ie OrderID 1 with ItemID1 can have 100 status's), your second query would need to look at all 100 rows in IDX2 to get all of the ones that are "open". But if it used IDX1, and only 10 of the 100 status's were of type "open", it would only need to look at 10 rows as it already has them in order.
IF an orderID and ItemID can only have 1 possible status, then they would do essentially the same thing on the query side, but the optimizer may do things a bit differently as it doesn't know that there is only 1 status per orderID+ItemID.