can I trade in two apple products at same time？

You can trade-in as many qualifying devices as you wish - either in-store at a physical Apple Store, or using the online trade-in process:

Apple Trade In - Apple

Each device is assessed its own right. If using the online trade-in, you will be sent a shipping kit for each device. Devices are returned, assessed and a credit value assigned. The trade-in value is returned to you by way of an Apple gift card for each device - and these can be used to purchase whatever you wish with Apple. 

If a trade-in is initiated during an online purchase, you will usually pay full price for the item - but will receive a refund against your credit card upon subsequent return (and acceptance) of the item that was returned for trade-in.

Many choose to complete their trade-in in-store - as doing so removes any uncertainty as to the trade-in value or acceptance.

Be aware that if a trade-in is initiated during an online purchase, you will only be able to trade-in a single device for that transaction - although you can also apply trade-in credit (Apple Gift Cards) for any other devices that have already completed the trade-in process. By contrast, you can trade-in multiple devices at an Apple Store for an in-store purchase.

The easiest method is to simply trade in those devices for Apple Gift Cards, then to use them towards your new iPad. 📲

Good luck! 👋🏼😉