I have 2 IMACs.Can I trade them both in when I buy my new IMAC?

you can trade in a wide range of different things for credit, if it was you then I would head on to this page and read up

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

You can trade in one first for an Apple Gift Card and the other at the time of purchase of the new Mac. You should be able to use the Gift Card as part of the purchase. I'd verify that but it should be possible.

You should address that question directly to Apple. We are a forum of Apple users.

Not possible unfortunately.