Orders are not automatically merged.
If you have made two separate orders, they will be shipped separately, and you will pay the shipping fees twice. If you wish to merge the orders, you must request one of your order to be cancelled and refunded, and the other order to be modified with the item added. An invoice will be sent to you and your order will be shipped when your invoice has been shipped, on the next fulfillment schedule.
To know when your cancelled order will be refunded, check this article: https://support.pulsar.gg/a/solutions/articles/67000709551

You must make your modification request before your cancellation deadline. To know when your cancellation deadline is, please read this article: https://support.pulsar.gg/en/support/solutions/articles/67000701701-how-can-i-cancel-or-modify-my-order-