When I am putting or accepting an order with 3 quantities: B, rr and rb, another order with 0.5B, 2rr and 2rb is almost same to me. The only difference is the latter has a leverage of 2 times.
They have very different exposures to exchange rate risk though. The market rates will be the safest, so demand should cluster at that point. I accept it's not perfect though. This may be an argument for the preferability of a Ripple-esque system without collateral.