Regarding the dual-valve setup for the drain pipe
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There are two issues: (1) What type of valve is suitable for use when dual valves are installed at the drain pipe. Are they all stop valves? Are they all gate valves? Still segmented by gate? Why? (2) When using one valve per section, which one is in front and which one is behind?
Generally, it is the front gate valve, used for shutdown； Rear shut-off valve, controls the discharge volume.
Generally, gate valves or ball valves should be used upstream for shutting off flow; they remain open during normal operation. Downstream, a throttle valve is used to control the flow rate, and a needle valve can suffice when the flow volume is low. In the absence of such options, a globe valve can also be used as a substitute
I think a stop valve and a gate valve would be more appropriate