Explain this quote: "doubtful it stood/as two spent swimmers that do cling together and choke their art." What literacy device would it be?

Shakespeare uses a simile in the bloody soldier's report from the battlefield to explain the struggle between the king's army and the invading Norwegians and their allies, the Scottish rebels. The comparison is based on the image of two tired ("spent") swimmers who hold on to each other to keep from drowning. The result, of course, may be that both "choke" and drown.

The soldiers of the two armies are exhausted, so much so that the outcome of the battle is "doubtful." Neither seems capable of winning; they "choke their art," meaning they can hardly fight. The soldier wants the king to know that victory looked unlikely until Macbeth exerted his leadership.