One hose connects to the area around the fuel filler so that when you over fill the tank, it has someplace to go (open fuel cap - look in recessed area, you should see some sort of hose). This hose is only used when the fuel cap is open.
Some bikes vent through the fuel cap, some have a vent hose. This is to allow air into the tank, it is not an overflow. I would not put a vent hose into a catch tank, especilly on fuel injected bike. In heavy rain (like Barber last year) water can get into a catch can. On a carburetted bike, the tank can create a vacuum and act as if it is out of fuel, a fuel injected bike can pull water up the vent line into the tank (don't ask). I just pulled the 2 hoses off the bottom of the tank on my RC and SV.
