One of the main methods for obtaining mixed fermentative acidogenic inocula is the isolation of spore-forming bacteria (SFB) from different mixed matrices. SFB have proven to be suitable inocula in Dark Fermentation systems, although they have certain limitations. In the present study, the formulation of a mixed consortium is studied, by combining SFB and Water Kefir (WK) in Mixed-Yeast-bacteria (MYB) fermentations tests. First, both inocula are properly selected and maintained. Then, a scan of different pretreatments is performed on stale bread that was selected as retrieved substrate for the MYB tests. MYB tests are conducted in batch mode, under anaerobic conditions, using the basic-pretreated stale bread. Systems inoculated with independent inocula (either SFB and WK, in a 10% v/v) and combined (WK+SPB, 5+5% v/v) are studied in a three-cycles compatibility experimental campaign. The gas production and yield, its composition and the liquid titers of organic acids and ethanol are measured to shed light on the fermentation dynamics in each case. The adaptation cycles resulted in progressively reduced lag phases for all systems, and interesting results were obtained for the WK+SFB system, which exhibited higher hydrolytic activity and achieved the highest hydrogen, ethanol and organic acid productivity.