I’m not sure about Stanford, but some colleges assign applications from the same HS to the same admission reader so s/he can compare the applicants more fairly and conscientiously, For other colleges, the comparison may be made in the committee if both of you make it that far. No college sets an admission target for any HS consciously, but all of them would limit the number of admits from a single HS, consciously or unconsciously, in their effort to shape the incoming classes.
REA to Stanford (unlike Harvard, for example) seems to offer little advantage. Moreover, Stanford also doesn’t seem to want to reread an application (unless you’re a legacy) a second time (even in the RD round). That appears to be the reason they reject, rather than defer, most REA applicants. Unless you’re a clear admit (with a significant hook, for example), you may want to reconsider whether you should REA to Stanford.