Two air exhausts into one duct

Right now I have one grow tent exhausting air out through a chimney with a 70 CFM fan. I'm planning on building a slightly bigger tent to place alongside it and using a 160 cfm fan to exhaust air, but I am not sure where to exhaust yet. The easiest option is to join the new exhaust with the old on and have them both go out the chimeny. Will the stronger fan create a back-pressure against the weaker one, reducing it's air flow?

Any other problems you see with this setup?

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" Will the stronger fan create a back-pressure against the weaker one, reducing it's air flow?"

Yes.

I have a similar set up with the air from my charcoal filter going into my hot air heating system. When the stronger furnace fan comes on there is back draft up into my air filter system. I installed a back draft damper to close off the air filter ducting when the furnace comes on, but it wouldn't work with both fans on all the time.

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you can move the fans so they are both after where the the exhausts join... then use a damper to control air flow