BPMN21 — number of start events for sub-processes

The specification defines that the type for start-events of sub-processes can only be the none-type... However, the allowed number of start events for sub-processes is not mentioned... at least I couldn't find it throughout the whole specification.

As far as I understood BPMN, two none-typed start-events in sub-processes would not make sense but the point should clearly be described in a specification.