It's 100% possible. I have a game on Itch and Steam (it was released on Itch first), and I give Steam keys for Itch purchases. Steam keys are free to the developer. The only thing you'd lose is a bit of time setting it up through Itch, which really isn't much :) I wanna buy this, but I also want guarantee of a Steam key when it comes to that platform. Thank you!
Here is the best I can do:
"if Steam keys are free to me as the developer when/if Nox Archaist is released on Steam, I will provide you a steam key at no cost if you purchase the game on itch.io"
The main hedge in there is the "when/if" because I have never donea Steam release, so I don't know what I don't know. I've read their FAQ and it looks like a $100 fee and a bunch of work setting stuff up and that doesn't concern me....but you never know when you will find devils in the details.
I appreciate your interest!
Also, you get that $100 'fee' back from Valve as long as you sell as least $100 worth of your product. Might be worth getting a page sooner than later to get folks wishlisting and for marketing purposes. Just an idea.
Thanks for all the answers!!