I have 2 series-connected water heaters. I have sediment showing up in my shower head clogging it. So I am thinking I need to flush my water heaters. I understand if I have 1 water heater to shut off the inlet valve and drain the 1 unit and flush. However, I have 2 and they are in series. There is only 1 inlet shutoff valve that goes to the first water heater. Do I drain and flush the first water heater then try and drain and flush the 2nd (downstream) tank? I can see how to drain both but not sure how to flush the 2nd tank (downstream).