Green mould, caused by Trichoderma aggressivum, is one of the major fungal diseases of button mushrooms. The main problems in chemical disease control include a lack of effective agents, the occurrence of pathogen resistance to pesticides, and the harmful impact on the environment. In an attempt to find a solution, the interaction between two beneficial microorganisms, Bacillus amyloliquefaciens B-241 (an antifungal agent) and Streptomyces flavovirens A06 (a yield stimulant), was investigated in vivo. The synergy factor (SF) was calculated as a ratio between the observed and expected impact on the yield or efficacy of disease suppression after artificial inoculation with T. aggressivum. The highest control of T. aggressivum was achieved by joint application of the two beneficial microorganisms. The additive interaction between microorganisms in efficacy against the pathogen was revealed. The largest yield was obtained in mushroom beds sprayed with the two beneficial microorganisms combined (B-241 80% and A06 20%). Regarding the impact on the yield, synergistic interaction between the two microorganisms was confirmed (SFs were 1.62 or 1.52). The introduction of optimized microbial combinations could create new possibilities for biorational edible mushroom protection, with improved yield and quality and reduced risks to human health and the environment.