for two bottles you alternate refreshing your mix each week. Think of it like soda.
If you have one bottle it is carbonated, going flat, or flat.
If you have 2 bottles and you mix 1/2 your cup from each bottle, then as one bottle is going flat you mix it with 1/2 a cup of the new bottle...well, you get the idea.
You should end up with a range maybe in the 50-75% of max range instead of 0-100% for co2 production because one bottle always balances out the other. Those numbers are just made up and honestly if you are doing a new batch every week I don't know if there is going to be all that much fluctuation. When I say I don't know I mean I don't know! Either way with 2 bottles you will def have more co2.
for organics i found that even though my filter was working fine and flow was still good it had a lot of trapped debris. if I gently clean it bi-weekly the bacteria is fine but the amount of organics, or as i call it-crap, is greatly reduced. for gravel tanks a gentle vacuum is good, just going along the surface without actually disturbing the gravel will remove loads of said crap.
this is just what seems to work for me and others on here as well, but I don't claim to be an expert so take my advice for what it is and no more
20L My Other Tank: OEBT Shrimp
40B Inlaws Tank: 2 goldfish
75 Main Tank w/ sump: 5 Glossolepis pseudoincisus, 5 Melanotaenia boesemani, 10 Melanotaenia praecox, 5 Melanotaenia lacustris, some cories, a bn pleco and rainbow fish fry