Hi, I have 2 toilets on my boat, both with electric macerators pumps and both hooked to the holding tank. It seems that whenever we flush the toilet on the back of the boat, the front one gets filled, as if a return is coming back to it. I changed the joker valve on both toilets but it still does it. does anyone know if there's some "no-return" flap or valve that I can install on the tank or tank valve? or maybe it has something to do with the angle of the hoses? or something better than a joker valve, which is the only thing i've found so far?

Thank you!

Those valves aren't the greatest as they quickly form a coating of hard crystal like waste or bits get caught in them. You could install an in-line check valve but go a size bigger and adapt to the existing line it's installed on. You'll find these still need to be cleaned regularly though.

Also check the waste storage tank has a vent. No vent means the pressure will go elsewhere.