25 square meters then that would be 5 meters x 5 meters, that would be a medium sized room. In that case, the rooms would be approx. 16' X 16'. depending on the height of your room(I'll assume 8ft). Volume would be 16' X 16' X 8' = 2,048 cubic feet. Depending on the exhaust fan/ carbon filter and number/placement of the exhaust fan/ filters, you could be good. But you would need some spec's on the exhaust fans.
Figure 2,048 cubic feet x 5 = you need to be able to move 10,240 cubic feet per hour PER room. Just get enough exhaust fans/ carbon filters (one if you have a large enough diameter filter/fan), in order to figure out this you'll need to do a little research on fans/ filters that you are willing to purchase. Get the specifications on them and do a little math. You'll also want good air circulation, otherwise getting this much equipment may be not efficient.
I have never done this before, but in theory, it sounds correct to me.
BTW, fans and filters both have spec's for CFM, cubic feet per minute, this will be the focal point. And you don't just want a box fan, you'll need something that is contained(sorta works just like a turbocharge on a car). A fixed point in which there can be consistent and continuous air movement, being forced through the carbon filter. But not past the carbon filters spec'd CFM rating.
To answer your questions:
How effective are Carbon filters? They can be very effective, if setup properly. If it isn't setup properly, you may be unsatisfied with the results.
If I was growing in two rooms, each about 25 sq/m (16 sq feet) and filtered 5x the room volume every hour would it be a problem? No, not if you set it up correctly.
