Is it possible to get this game backwards compatible with online co op??

It was a great couch co-op game. Too bad the rest of the series won't be backward compatible.

Yes the game is backward compatible on the Series X. The full list of games that are can be found here https://www.xbox.com/en-GB/games/backward-compatibility, and yes it is a 2 player co-op game.

I'd definitely like a few more Resident evil games on backwards compatibility. Preferably one with co-op too.

If you're looking for a good backwards compatible game.

Portal 2 is excellent for a little co op game. Great problem solving puzzles

Its more than possible, its real. If you've got a game that has multiplayer or co-op and you're playing it online on the Xbox 360 you're bound to have run into an Xbox One player at some point. Assuming the game is something with backwards compatibility
features. The Xbox One runs what's basically a simulation of the 360's UI to play backwards compatible games. However, its impossible to play two different versions of a game. You can't play a title that was released on the 360 and the Xbox One and expect
to be able to play it with the person who has the Xbox One version. So basically you can play 360 games online or offline on the Xbox One but not the other way around.

Here's about backwards compatibility. https://www.xbox.com/en-US/xbox-one/backward-compatibility

In short, can you? Yes. Just only on Xbox 360 games that are compatible with the Xbox One.