Personalised products are engraved at point of manufacture (often China) - and are then shipped directly to the customer. By contrast, non-personalised devices are instead fulfilled from in-country stock - often held at one of Apple’s regional fulfilment centres.
As such, you might anticipate that your iPad and Pencil will be shipped separately.
Apple’s estimated shipping dates, although not guaranteed, are generally a very reliable indication.
Assuming that your ordered online from Apple, you should have received an order confirmation email with your order reference. You will also usually receive an email when each element of a split-consignment ships to you.
You should also be able to access your order online - where you can verify the current status. You typically see the order status change to Preparing to Ship - 24-48hrs before shipment occurs. When shipped, you may receive a tracking email.