Stack two fans for better air exchange.

I'm having trouble with heat. My one 400 cfm 6" fan isn't enough. Can I stack them one on top of the other. Will that work.

I don't understand how you mean?

Assuming both fans have same speed.

If you connect two inline duct-fans in serial, you get twice the sucking-power - but same airflow-speed (can handle bigger CF's).
If you connect them parallell, you get twice the airflow - but no increase in power (airflow-speed).

I'm not sure I understood you question.

What I mean is I have my 6 " filter on top if my carbon filter and then ducting from the fan to the attic. If I get an identical fan and put it on top of the other fan and then ducting will the suction power be doubled or will it ruin the fans. So its carbon filter, fan, fan, ducting

What I mean is I have my 6 " filter on top if my carbon filter and then ducting from the fan to the attic. If I get an identical fan and put it on top of the other fan and then ducting will the suction power be doubled or will it ruin the fans. So its carbon filter, fan, fan, ducting

Could do. But you'd get a lot more efficiency by putting the second fan on the intake instead of doubling the out take. Even better still if you hook them up to the same speed controller.

Remember one thing - the faster the air moves through your carbon filter the lower the odor removal efficiency will be... To ensure you are giving your filter a chance to do is job, keep your filter dwell time at .1sec or better... Dwell time= the amount of time the air stream is in contact with the carbon bed.