can I use ipad pro 12.9 6th gen as a trade in for apple watch

You can trade-in qualifying devices as you wish against an Apple device purchase of your choice - either in-store at a physical Apple Store, or using the online trade-in process:

Apple Trade In - Apple

Each trade-in 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, the online process assumes that you are trading-in and purchasing the same type of device. Many choose to complete their trade-in in-store - as doing so adds flexibility in trading-in a different type of device against a new device purchase - along with removing any uncertainty as to the potential trade-in value or acceptance.

For as far as i know, this is possible