2 filters ok

So I was occasionally smelling some whiffs of green so changed out old filter with new one, then to muffle the sound outside grow room, I attached the old filter. Sound is less air flow is still good, no smell, fan/variable switch, nothing is any warmer than when only one filter installed, so question - am I hurting anything ?

Thanks in advance

The only thing you could possibly be hurting is your inline fan, and you can probably expect it to give out a little sooner then it normally would

If both of the filters are rated for the fans cfm then I see no problem.