Woah, sloww down everyone. First off, what kind of fan are you using? You have the right order in your setup, but the important factors are your temps, how powerful the fan is, how big your tent is, the length of your duct maybe, and are the room and attic ventilated to pull/push enough air?
I use a smaller to medium sized tent with an inline fan that pushes 180 cfm or so. My duct run is around 25-30 feet of 4", pulling from inside the tent, hood, duct, fan, duct, exhaust, and I have excellent airflow. I have an air-cooled Sun System hood, and you don't need two fans if your tent isn't huge and you're not doing a huge grow op. If you had a room, or a big tent, with multiple air-cooled hoods and etc, then two separate fans would be more realistic... one to in/out across the hoods to cool them and one to in/out the tent environment for the plants. I've been working on a new filter design of my own to ensure that I don't lose much throughput after adding the filter when I switch cycles, but at the moment my temps stay in the seventies pretty much all of the time.
As long as your tent is exhausting air slightly faster than it pulls air in, there's going to be a vacuum. If your fan is powerful enough it will still create that vacuum even after pulling through the carbon filter. That vacuum is nothing more than negative pressure guaranteeing that any smell from growing won't escape your tent from anywhere other than your filter/duct/exhaust. If your tent is in a room that has enough air coming in through cracks, under doors, from windows, etc. then you shouldn't have any problems.
If your fan is powerful enough, but you're experiencing issues, it could be that the room where the tent is located doesn't have enough air coming in (which means you have to add an intake, from outside for example) or it could mean that your exhaust is obstructed in some way (meaning your attic isn't venting outside or something on that end).
No you don't need more than one fan unless the duct run is really long, or your house is sealed really tight. For air to go out, air from elsewhere has to come in to replace it, so if your place is sealed super tight, you might need an intake duct running outside, but even then you wouldn't necessarily need a second fan.
Also, I'm not sure what you mean about the attic and garage, but damn near every attic is vented to the outside somehow. I'm sure there are vents under the overhangs, ridge vents, dome vents, or something along those lines, but it's something to confirm as previously stated, since mold in the attic is a bad deal. With a small grow I would guess that this shouldn't be an issue though.
