The specification says:
10.5.2 Start Event All Flow Objects that do not have an incoming Sequence Flow SHALL be instantiated when the Process is instantiated.
There are some exceptions to this rule, but they don't touch the following discussion. Since Events are Flow Objects, this applies also to Start Events: When the process with multiple Start Events is instantiated, all Start Events of this instance are instantiated (i.e. they get a token).
Further down it says however:
There MAY be multiple Start Events for a given Process level. Each Start Event is an independent Event. That is, a Process instance SHALL be generated when the Start Event is triggered.
That means on instantiating a process, each Start Event gets instantiated, which in turn instantiates a new process. I don't think this was intended.
In the same section the semantics for calling global processes is defined differently (without mentioning it as an exception to the above rule):
If the Process is used as a global Process and there are multiple None Start Events, then when flow is transferred from the parent Process to the global Process, only one of the global Process’s Start Events will be triggered.
So called global Processes are treated differently from Subprocesses (I know, somewhere else it is stated, that a Subprocess can only have a unique Start Event - a rule that is contradicted in the immediately following sentence). I think this is confusing.
Change the above sentence
All Flow Objects that do not have an incoming Sequence Flow SHALL be instantiated when the Process is instantiated.
This way, multiple Start Events are always alternative. This would be in line with many other sections of the specification:
13.2 Process Instantiation and Termination A Process is instantiated when one of its Start Events occurs. Each occurrence of a Start Event creates a new Process Instance [...] Note that two Start Events are alternative [...]
13.3.4 Sub-Process/Call Activity If the Sub-Process does not have incoming Sequence Flows but Start Events that are target of Sequence Flows from outside the Sub-Process, the Sub-Process is instantiated when one of these Start Events is reached by a token. Multiple such Start Events are alternative. [...]