hi,
i think it will not work, I tried to install door station and monitor over one cable with the help pf spliter and poe Extender and it seems that both devices need all 8 pins in the rj45 cable so you can't split them. The poe extender will only work for IP camera.
But I'm not an expert on this I just could not connect over POE extender. But the dahua video monitor has wifi? If you can connect the power over electricity (12V) adapter?? then you don't have to split one cable.