For a 48 gallon, you might need even more than the 2 2-liters. I'd say start them together and see how much co2 it provides for your tank. Do an advanced search for DIY co2 mixtures. There are tons of opinions on that issue
I'd say those refill packs are overpriced yeast, and stabilizer is baking soda?? Whatever it is, yeast is cheaper from the grocery store, and is effective.If you don't mind spending a couple of dollars, Petsmart has CO2 refills 3 per pack for around 7 bucks. Last 3 months per mixture. I currently use it on one of my 30g, works well.
All you need is pure cane sugar, and add in the petsmart mixture.
Here is a link for the complete kit, however you will only be needing the refills.
Refills can be had at;
As far as starting them, I would start one, then one week later start the second as they deplete you will have about one week to start a second batch without interuptions.
DIY fermentataion normally takes about a day or so to get going, depending on room temp etc..
OK I just tested my Bubble per minute coming out of the end of the tube and it was 28. I took off the airstone to get the count. Is it me or is this to high? could it cause damage to my fish.