Hello...
A grammar book says that the sentence A is incorrect.
A: *They’ve given the job to Max, who has no qualifications, who starts next month.
Then, how should A be changed?
Maybe like B below, adding "and"?
Or...
Maybe like C, without the "commas"?
B: They’ve given the job to Max, who has no qualifications, and who starts next month.
C: They’ve given the job to Max who has no qualifications who starts next month.
Join them with "and". No comma. They’ve given the job to Max, who has no qualifications and who starts next month.
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Join them with "and". No comma.
They’ve given the job to Max, who has no qualifications and who starts next month.
You can also rearrange it even more.
They've given the job to Max. Unfortunately, he has no qualifications and he's supposed to start next month.
CJ
Your book is right: this so-called 'stacking' can only occur with integrated (restrictive) relative clauses. Your example is of the supplementary (non-restrictive) kind, so it is ungrammatical.
The problem is that a supplementary relative clause can't combine with its antecedent to form a larger element which is antecedent for a second supplementary relative clause.
Your suggested