Don't know why they would. Petrohawk doesn't need any Barnett acreage (have more than they can handle elsewhere). Devon already owns big leases in both places. I don't think either needs any help drilling. Just don't see what either party would gain on this.
It's possible that they are going to allow each other to use some leasehold to drill on acreage they own. Deals are made like that all the time. Haven't heard of any large-scale partnership though.