Multiple room exhaust questions

Haven't found a definitive answer yet, maybe you guys will know. So I've decided to upgrade to a perpetual grow, which will involve a 4x4x7 1000w (actual power) flower tent and a 3x3x7 400w HID for veg and 48w fluorescent for clones.

I'm currently venting straight up a chimney, but the chimney only has one port and I don't want to modify it any. Could I simply use a 6x6x6 Y duct and combine the two exhaust lines AT the chimney vent? Would the flow be hindered too much? It's basically forcing twice the amount of air into the same sized exit.

Any other ideas? I have a perfectly good chimney and don't want to make another exhaust anywhere else if I can avoid it.

Thanks!

make sure the fans powers match up or you may get suck or blow where your not expecting

Also the 150mm pipe will limit how much air you can move but adding the 2nd fan should still move more air getting you more air exchange and better heat reduction

You could patch both fans into 200mm ducting and then drop down to 150 at the exit if you have to that might help a bit, but probably not