Hi,
I have a similar situation with 2 EX1200 filters and an AM100 reactor (250L tank) on one of the returns and I have often pondered this same scenareo. I haven't come up with a solution yet though but would love to have the option of fitting spraybars along the back of the tank pointing fowards and still achieve even co2 distribution. Maybe the solution is to have 2 reactors aswell, more expense clutter and complications, yuk, which is what puts me off.
What I have at the minute is the spraybar (the complete width of the tank) from the non co2 enriched return level just under the water with the jets pointing along the tank aided by a koralia 1 to push water along the surface and the other co2 enriched retun ( again a spraybar the complete width of the tank) at the other end of the tank about 6" below the surface and pointing downwards at a slight angle of about 30 degrees from vertical. This is aided by a Koralia 2 fitted above the spraybar pointing downwards and at roughly the same angle, thus forcing the co2 enriched water straight to the bed of the tank. Both Koralias are on the same timer as the co2 solenoid.
This arrangement causes a circular movement of water along the length of the tank and works ok albeit not ideal (i.e. back to front) and all that hardware doesn't look pretty. The fish though seem to like a steady flow accross the substrate and constantly 'play' in this flow which is more akin to that of a natural riverbed than the flow that would be achieved from a back to front scenario.
Regards, Chris.