Overflow bottles are always a good idea as they increase utilization of your rad.
Any I've seen with 2 hose fittings have one as the hose to/from the rad, and the other as an overflow in case the bottle gets full so it can puke on the ground. If you look inside these bottles one goes right to the bottom, and one would be near the top. The one at the bottom of the can is where the fluid goes in, and when the fluid cools off, since the cooling system is a sealed system, it produces vacuum and sucks cooling back into the rad until either the bottle is empty or the rad is full.
The hose at the top of the tank is for overflow in case it's needed. If you have a can with only one hose fitting it needs to overflow through the cap or fill area of the can (whatever you're using).
Since you use it as a frequent driver I would definitely recommend that you use an overflow bottle. Having the rad full at all times (not just when it gets warm) will help with cooling. I hope this helps.
