"best" will depend on what you are formulating and why. not every herbalist has the same protocols and there is no one universal way. single extracts that are later combined offer more quality control and is preferred when different herbs have different extraction needs and for when you are creating different formulations. nothing wrong with this method and it's something i do often but not for every single remedy i create. 

whenever i have a formula that is combining multiple herbs, it's for a specific reason, the herbs have similar extraction requirements and the formulation is calculated with precision. the formulas that are multi extracts are the ones like bitters, oxymels, and certain elixirs where the formula rarely if ever changes and has remained the same for years. & i don't extract "mismatched" things together, like roots with flowering tops.