Is a single 6" 440cfm enough for two 4x4x7 tents with a 4" carbon filter in each tent, connected to a 4"-6" Y joint connected to the fan connected to outside?
Well the most basic calculation is 4 x 4 x 7 = 112 cfm x 2 filters = 224 cfm needed for air volume alone. Factoring in filter resistance and hood resistance is dependent of the products you chose, so look into their manufacturing specifications to complete the necessary calculations. You'll also need to factor in the length of ducting, frictional losses, total pressure loss, (balance by design), barometric pressure, elevation, humidity, and room temperature. That is, If you want to be mathematically accurate. This webpage has the necessary information you're looking for Air Flow, Air Systems, Pressure, and Fan Performance Or you can just do it the lazy way and assume it'll be enough. I can tell you now balancing the two is more work then its worth. Buy two blowers and run one to each tent with a fan controller @ 50-60%.