If the jobs were separate bids, then each contractor should write their own contract-that would be the best way to make sure both jobs are coordinated by their respective contractor as far as time, delivery of materials, labor etc are concerned-and whatever you do, please don’t try to be an on-site supervisor of the job-if you want to do that, be your own contractor and hire some laborers to supervise yourself-you contracted with others to do the job because you don’t want to be in the middle of it-good luck with your wall-hope it looks great...
the reason I don’t want to hire my own laborers is because of injury liability.
I will get landscaping guys to do small stuff but I will test them first on smaller jobs just to see that they are honest.
(and since this is California, I take a risk every time I do even that...)