These words are uttered by the sergeant who returns from the battlefield to inform King Duncan of the outcome of the battle between his army and the invading Norwegian army. The sergeant compares the two sides by employing an image of two fatigued swimmers. Each swimmer struggles to prevent himself from drowning. They are both exhausted, and they attempt to defeat each other in order to save themselves from death. The adjective "spent" means that both of them are drained, and their "art" refers to their swimming skills. However, neither of them can prevail, and they are faced with the threat of death. The literary device utilized by Shakespeare in this part of the sergeant's speech is a simile. This is how the sergeant describes the battle and the unpredictability of it until Macbeth arrives. The sergeant then talks about Macbeth's valor and states how he killed the rebel Macdonwald:
Till he unseam'd him from the nave to the chaps,
And fix'd his head upon our battlements.
As a result, Macbeth is promoted.
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.
swimmers who stay together even though they choke and drown each other. "two spent swimmers, that do cling together/And choke"