the thing that they should do to prevent fatigue and maintain the quality of chest compressions is: Change positions every 2 minutes.
Giving a CPR require someone to stay in one place and doing the same movement over and over again (such as chest compression)
IF we do a CPR more than 2 minute, our body will start to feel fatigue that could decrease the quality of our CPR. If we have 2 persons doing a cpr, changing position every 2 minutes will help maintaining this quality.