Static + Dynamic directional lighting, how? (can't have stationary)

How can I set up directional lighting so that I have terrain casting dynamic shadows from dynamic directional light and everything else casts static shadows from static directional light (I cannot have stationary lights, so it’s either dynamic or static) ?
I could have all static lighting, but device’s RAM is only 8Gb, so lightmaps + everything else just doesn’t fit into the RAM.