One pump will do it if you batch sparge, During the mash it would pump from the bottom of the MLT, through the HERMS coil, and back into the MLT at the top. Then it would drain the MLT into the boil kettle, then pump sparge water from the bottom of the HLT into the top of the MLT, then drain the MLT again. Then it could, if desired, do a whirlpool in the BK by pumping from the bottom of the BK into a whirlpool port mounted higher. Then drain the BK. Two pumps makes for fewer plumbing changes, and would also make fly sparging possible. You can get some control ideas from the links in my sigline.