I already have a license with STIM
Since SAMI and STIM represent different rights holders, you must pay copyright compensation to both organisations when you play music in your business. SAMI represents the artists and musicians who perform on the music. STIM represents the authors who write the lyrics and the music. Even if SAMI and STIM were a joint organisation, the cost would be the same, since they are two different groups who are both entitled to compensation separately.