Trade in two iPhones towards an iPhone 11

You can, but it cannot be done in one step.

The Trade In option in the online store only accepts one device at a time.

You can trade one of the iPhones in using the Trade In option outside of the purchase process, and then once you get the gift card for it, you can use it during purchase of the 11 in conjunction with the trade in.

So start here to trade in one of the iPhones on it own: Apple Trade In - Apple

Then when you have the gift card, go through the normal trade in and purchase process with the other phone, and enter the gift card code during checkout.

see this link:

https://www.apple.com/shop/trade-in?afid=p238%7Cs4wibFo6O-dc_mtid_1870765e38482_pcrid_382066772864_pgrid_80259604234_&cid=aos-us-kwgo-brand--slid---apple+trade+in-e-product-