This is an age old question that comes every now and then. First in reference from the BMBL (Biosafety in Microbiological and Biomedical Laboratories), a clear 12-inches (305 mm) should be present for separation of materials whether clean to dirty or between two users. “Therefore, as a general rule of thumb, keeping clean materials at least one foot away from aerosol-generating activities will minimize the potential for cross-contamination.” Second, if the work contains any volatiles or vapors, these would present exposure between the two operators as they are not retained in the HEPA filters. Lastly, use of proper aseptic technique is always important for the highest level of both product and personnel protection.
A few additional comments, most E, H & S personnel would not advocate for two technicians work in the same cabinet. This perspective is because the two technicians most likely would be doing different research work and would be too difficult to control work practices, etc. to assure complete separation. If work is done with two technicians in a 6-foot Biosafety Cabinet (BSC), the work should be similar and work practices need to be adhered too!