Two part base nutrients typically come in a Grow/Bloom or A/B formula. This allows grower's some control over the ratio of different nutrients available to the plant at different growth stages. For example, plants in their vegetative stage can have a higher ratio of nitrogen, and plants in fruiting or flowering stage can be given a higher amount of phosphorous. This should yield better results for plants that have different growth stages than a single bottle would. Some brands have a Grow & Bloom formula, given separately depending on the plants growth stage, while some brands use an A & B formula that require both nutrients throughout the plants growth cycle but in different ratios depending on growth stage.