I've played with the same drummer since 2009. We've done some practices together just the two of us when we want to figure something out about a song that's bothering us and it's always been beneficial.
Did it work? Well, we haven't killed each other yet, so I guess it did! But in all seriousness, I would definitely say it helped foster a "rhythm section as a coherent unit" mentality. Maybe time alone did that, who knows?
Hints? Keep it light hearted, but have a purpose - like working on a song or portion thereof in particular. I'd say going into it entirely rudderless will amount to playing 2 songs then spending the rest of the time drinking tea and talking bollox.