Double-spines look cool indeed - another vote there.
Perhaps to integrate a rider you could devise a cool saddle that sits into/around the shoulder area rather than on the neck behind the head. I'd imagine a rider armed with lance (or dragonlance ... hehe) would make his attack after the dragon had already done his thing as they swept past on a strafing run down the lines of the enemy army.
I can even see the rider issuing a , 'He' mine!' command as they dive into attack and the dragon angles it's dive so that the rider make his own attack at his chosen enemy while the dragon chews on a few nearby minions.
I can still see a rider on this dragon - they'd be lifelong companions rather simply rider and riding beast (at least in my head anyway).
I always find myself smirking at what I write when making up rationales for fantasy critters
- but it's so much fun!