Please explain the following quote from "Macbeth": "Doubtful it stood, as two spent swimmer that do cling together and choke their art."

qui0006,

In Shakespeare's tragedy "Macbeth," there is a constant conflict between good and evil.  In this opening scene, that conflict is revealed through a Sergent's narration of what happened on the battlefield.

The Sergent is bloody and weary from battle as he obviously is showing "gashes" that make him feel "faint."

I cannot tell--but I am faint.

My gashes cry for help". (I.2.42-43)

The subject and themes of strength, injury and completeness area  common thread throughout the playing, could make a great deal of the injured condition of the soldier. 

Some of the language may be ambiguous, but ti is rich and vivid nonetheless. There is a conflict between "merciless Macdonald" and the troops loyal to the king. This conflict "stood doubtful."

This conflict is put into a form of an analogy between, ..." two spent swimmers that do cling together/ And choke their art." Instead of merely saying that both sides were evenly matched or there was a stalemate, Shakespeare uses imagery from swimming in which most viewers (readers) would be familiar with.

It is the story of the swimming contest between Julius Caesar and Cassius in the Tiber river as mentioned in the play "Julius Caesar." This image invokes tension, struggle, and exhaustion as one out-duels the other for victory.

The swimmers are "spent" (energy depleted) and they "choke their art (their swimming ability)." Their skill of swimming ("their art") compromised by the actions of each other, but they are "choking" each other by means of swimming whereby the other will not concede. They both ran out of energy to continue without the other giving up. They are both in danger of drowning as they wear themselves out  and "choke" each other.