You want approximately 2 Liters of CO2
generation for every 10 gallons of tank. So two 1 Liter bottles for a 12 gallon aquarium would be about right, as would three 2 Liter bottles for a 29 gallon aquarium. If you find that isn't quite enough, then I'd try to find 1.5 Liter bottles as a pair of 2 Liter bottles is probably overkill.
You can setup both CO2
bottles at the same time and then just change one out early, or stagger when you start them. Either way is perfectly valid. You'll want to fill the bottle 3/4 full or to the top of the label.
If you're looking for a simple diffusion method, you could feed the CO2
into the intake of the filter. This would allow the impeller to chop up the bubbles into fine bubbles and spray them about the tank. This can be noisy and will increase the wear on you impeller.