8) Again, not sayin' it can't be done. I can't swear to it but I seriously doubt Marineland would sanction the linking of 2 motorized, pressurized units. They possibly might option the 2 filters to be run in tandem (dedicated) but not in series. There is an option for the filter to be linked to a bio-wheel but that's passive (water driven not motor). Double chk. that manual of yours. I understand you're objective & it is a good one but the negatives far out weigh the the positives. The potential damage to one or both motors or at the very least, shortening the life of the motors & possible resulting flood potential is just not worth the 15 min. extra rinsing of the media once or twice a month. JMPO, "T"I am pretty sure that even in the owners Manual for my Magnum at least ( cant say about the Rena ) it says they can be run that way.
Receipe for disaster!! First of all the gph doesn't double in this case. The 2 filters operating seperately, depending on media content & packing might yeild 450/500 if you're lucky. In series, one negates the other. The contents of the 2 cans are not equal so the pressure is not equal. As dirt, detritus, ect. accumulates it becomes even more unequal. You would have 2 motors trying to move water at different speeds thru each other back into the tank (not good).look at it this way, water leaving the first canister (for example) is going 350 GPH, since the second filter would be moving the same, we'd hit a combined speed of 700 gph. That being said, you expect the second impeller to move at that rate of speed? Not only accept 350 gph at the intake, but also INCREASE that speed to 700 GPH? Essentially, you would achieve that 700 gph if you ran them by themselves, if there's to much flow, then make whatever the outlet is bigger, simple as that.
Canister filters are not 'fully closed' systems. Both ends are open.Gravity will siphon water out when the system is not fully sealed priming opens a valve to allow air to escape.)
I won't continue to argue the point, it's something you either understand or you don't