Interpretation, especially simultaneous interpretation requires huge levels of concentration . Normally, simultaneous interpreters will change over around every 30 minutes. Therefore, when simultaneous interpretation is required, two interpreters will be required for any meeting over 30 minutes. Consecutive interpreters can work for longer, up to around one and a half hours at a time but a second interpreter would be needed for any longer or if the subject matter is complex.
There may be some limited circumstances where a single interpreter might be suitable (such as a series off short, non complex, sessions with breaks in between. However, any agency routinely offering single interpreters is unlikely to be working with true professionals.