There are Merchants that use FedEx Smartpost or UPS SurePost to send packages to vPost USA Warehouse. Such services come with two tracking numbers.
In the event wherein physical package label become spoil due to unforseen event, warehouse will not be able to track the package using only one tracking number. Providing both tracking numbers will make it easier to track the package.
The two tracking numbers can be found at the upper left and lower right sides. See image below as guide.