Since this is a waterfall pump would a check valve on the outlet side be able to engage since any reverse flow would be air and not water? If this is an issue then the only solution would be to put the check valve close where it Ts off of the main so there is enough water to pull the check valve. Then again, there is probably enough water on either side of the pump anyway...
All check valves have springs so they are normally closed and only open when there is a positive pressure difference between the inlet and outlet of the valve. When the waterfall pump is off, the other pump creates suction so the pressure on the inlet side of the valve is lower than the outlet side of the valve (negative pressure) so the valve remains closed even if there is air on the other side of the valve. This will occur on either side of the pump.
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