If your tanks not completely level it's going to be more water on inside than the other. Good idea though they had water features like that in Sinagpore, people with brooms out there early morning cleaning up algae and people trimming the grass...
It would be an awesome tank though. Something to put in your wall maybe :
Edit:
Also maybe the splash when it goes from tank to the sump? Also if it's saltwater won't you have a lot of evaporation due to a high amount of surface area and a volume?
Edit:
Another thought, wouldn't it have an unsteady temperature? As as it slides down the wall of the tank it will be cooled? Fixed by having two heaters one in tank where the water flows in and one in the sump... Just thinking about this...