Congrats on your new addition! Lucky Foster, he knew where to go to find a good home!

You did not mention, are Marty and Harvey both neutered? If yes, that rules out one set of circumstances.

Cats will have a hierarchy. At one year old, it may be that they are just sorting out who is the more dominant. IF that is the case, you just have to wait for them to sort things out themselves. Keep a water squirt bottle handy to stop any intense fighting.

Trying giving Harvey "first choice" attention. That is, talk to Harvey first, pet Harvey first, feed Harvey first.

I am sure other PTers will have more ideas.