Can I trade in two devices

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.

I just got off the phone with Apple Support because I wanted to trade in an iPad Pro 10.5", and an iMac 27" Intel I7 2014. I was told only 1 device may be used under their Trade in program. I would not be able to trade in both.

https://www.apple.com/shop/trade-in