Two sequence flows terminating on a single activity

What exactly does it mean if two sequence flows terminate on a single activity? Must a token arrive on both sequence flows before the activity can start? Or does the activity start as soon as one token arrives? In which case, what happens if a token then arrives on the other sequence flow? Do we then get two instances of all subsequent parts of the process?

As a related question, is this correct?
http://www.sylvester-bradley.org/rowan/graphics/NoGateway.png

Or do I have to do this?

Thanks - Rowan
NoGateway.png
WithGateway.png