Certainly, tall sunflowers cast shade and also leach all the available water from the soil (a bit like leylandii). But I've had no problems in growing climbing beans up tall sunflowers.
The trick (I find) is to sink an empty cola can beside the sunflower when you plant it. When the sunflower is some three foot tall, pull out the can and replace it with a bean transplant. (That way, you don't disturb the sunflower roots.) Just keep both plants well watered and you get edible seeds plus beans - plus beauty!
Of course, both those plants should be grown in good light.
Another tip is to sink two cans beside the young sunflower and adjacent to the bean, one can being perforated at the sides and base. Keep the perforated can in the ground and keep it filled with water. Your watering problems are over.
I have some lovely pix to illustrate this but I can't seem to imbed jpg URLs here by the IMG method. (Though I have their URLs hosted at ImageShack.) Perhaps some technical wizard could enlighten me? Sorry for the digression...
