It's always better to have multiple bottles with staggered start times, in order to maintain a consistent rate over time.
I recently began DIY on my 75 gallon. Started out with 3 two liter bottles, staggered one week apart, changing out one each week. The recipe Im using lasts two weeks before starting a slow decline. I got dramatic results at first, but as plant mass quickly increased, and fert levels were adjusted to meet the higher demands, I soon found that I wasnt getting enough Co2.
I decided to add a fourth bottle, combine them in pairs, and change out 2 per week instead of one. In theory that should almost double the output. Instead of always having two bottles full strength, and a third winding down, there is now 4 bottles running full strength, all the time. I also wound up going with 3 liter size instead of the 2Ls. More water = more Co2, for a longer period of time, as someone pointed out above.
The difference has been astronomical. Drop checker went from dark green to light lime. Plants pearl like crazy now, before there would just be a little bit near the end of photo period.
Here is a build thread I made on the system. You may get some food for thought. I havent gotten around to updating it since going to four bottles - DIY CO2 for 75 gallon (and others) Build Thread / Performance Reports