I sent Aquatop a email about it and this is what they had to say about it.
I sent:
[FONT=&]Is it possible or do you see any issues with combining the intake and output of your CF400 canister into one input and one output. I am looking to set up a 300g tank…I would like the total of 18w of UV (2x9w) and I am looking at a inline heater that uses 5/8” hose like your CF400’s. But I will also have other canisters running on the tank so I would like to minimize the amount of intake pipes inside the tank. Could I use a T or a Y to combine the intake to say like a 1” intake pipe in the tank and the same for the output?[/FONT]
They sent:
[FONT=&]Greetings Mark,[/FONT]
[FONT=&]Great question![/FONT]
[FONT=&]As a hobbyist, anything is possible when it comes to aquarium keeping [/FONT]
[FONT=&]As an employee of AquaTop Aquatic Supplies, I regret to inform you that your warranty will be voided if you set the filters up in this manner and damage occurs.[/FONT]
[FONT=&]If there is anything further we can do for you please let us know.[/FONT]