Are you sure they are both closed all the time? They will close them periodically.
Clams require the same water parameters as SPS: strong light, strong and turbulent flow, pH 8.0-8.4, calcium 400-450 ppm, alkalinity 3.0-4.5 meq/L, magnesium 1200-1400 ppm, nitrates < 10 ppm, phosphates < 0.1...