How to Evaluate , Construction Site workers when they directly work under more than one supervisor?

You can do it simply by dividing the workers into groups then assign each group to each supervisor for a period of time of-course some other factors should be put on consideration while assigning the groups to the supervisors .

you can shift the group of workers to different supervisors with time like every6 months or one years this will help identifying the quality of workers by comparing the supervisors evaluations. also it will help your training department identifying the training needs and from the other hand you can keep the best workers and terminate the low performers.

Thank you 

• the worker it self should send the same report to  both of them
• the supervisors must discus and decide who that will be responsible to the worker (monitor/mentor etc). they should discuss to sure that  each of them will responsible to the same numbers of worker or less more, depends on their agreement regarding their abilities to do the supervision.

NEED HAVE  GOOD COMMUNICTION BETWEN SUPERVISOR AND SITE WORKERS,,,,,,,SUPERVISOR SHOULD WELL DEVELOPED SITE WOTRKS,,,

As Salam o Alaikum,

In order to evaluate the workers. One must observe the site activities done or on-going by that particular worker.

To evaluate the worker one must be fully aware of that workers capability and efficiency that we can find out only by giving them some task and compare with the quality and time taken to complete that activity.

We can also invite suggestions from the supervisor who are directly leading these workers but we cannot trust the suggestions% as the supervisor may or may not like some particular workers due to their nationality or language or communication skills. 

I hope that helps.

Comparing supervisors notes on such workers would help.  It may be possible that each supervisor may have a different rating depending on the worker's performance since it would be impossible for that worker to complete each job given to him by multiple supervisors for a given time period.  One would have to go with the majority unless one has been able to evaluate the worker her/himself at various points of time.