Newbie dump valve question

[jmkm2804]

On my trailer in order to dump the grey tank the black has to be open. Why is this? On all other trailers I see you can open the black lever and dump the black tank close it and pull the grey lever and dump the grey.

[tfmdgm]

IMO, It's piped wrong. Each valve should be on their respective tank side of the "Y". You should be able to dump either one separately.