Two dirt bikers- one in black and red, sporting number 64, the other in blue and white displaying a red '21'- round the corner of a tree-lined track.
There are two people and two motorbikes in this picture.
Sentence one gives a description of two dirt bikers, and sentence two describes two motorbikes.
Two bikers implies there are two people and two motorbikes.
Two dirt bikers are two people.   Dirt bikers who round a corner use two motorbikes.