There is a complication in using multiple relative clauses that is semantic rather than syntactic in nature. Your first example involves serial, coordinated relative clauses. I've chosen examples I prefer to work with:
*/? I know some people who work with toddlers and who like grapefruit.
I'd say this pairing of clauses would in most contexts be too semantically disparate to work.
I know some people who work with toddlers and who attend karate
classes.
This works better, the contrast being acceptable. These are {people who both work with toddlers and attend karate classes}
I know some people who work in computing and who could [therefore]
probably help you.
Here, the second clause follows on naturally from the first. If the optional therefore is included, the first clause is of course non-optional.
In the following examples, the serial clauses would not work:
*There are a few people who/m I've met and who know all about the
dextroboper problem. [and who like grapefruit ...]
It doesn't make sense to serialise such disparate statements
There are a few people, who/m I've met, who know all about the
dextroboper problem.
ie there are only {a few who know all about the problem}, and I've met them all: 'who I've met' is a parenthetical
There are a few people who/m I've met who know all about the
dextroboper problem.
ie amongst {the people I've met}, a few know all about the problem.
Looking at your second sentence,
I can name a few people who I'm friends with and who got a job in that
company.
is again a coordination.
I can name a few people [who I'm friends with] [and] [who got a job in that
company].
But in your
I can name a few people who I'm friends with who got a job in that
company.
there is not coordination, but sub-setting (nesting of clauses):
I can name [a few people who I'm friends with {who got a job in that
company}].
cf
I can name [some friends {who got a job in that
company}].
The 'who' can't be dropped in 'standard British English' in the first of these two sentences, and dropping it in the second sounds like an informal AmE usage.
If I were writing this, and my intention was to convey the idea that the participants' friends were told to order drinks, I would write:
"The friends of the participants, who were told to order soft drinks"
However, if my intention was to convey the idea that the participants were the ones who were told to order drinks, I would write:
"The friends of the participants who were told to order soft drinks"
It is still rather ambiguous in print. To avoid this, I might split it into two along the lines of:
"Participants were told to order soft drinks. Their friends ..."
or bring the subject and verb closer together, as in:
The participants' friends were told to order soft drinks
Best Answer
There is no grammatical rule against attaching relative clauses to other relative clauses in English.
In a highly inflected language like classical Latin, it is possible to insert a seemingly indefinite number of clauses to others, as anyone who has been assigned to translate a page-long sentence of Cicero knows. But English is weakly inflected, and highly dependent on syntactic markers like word position and proximity to deliver meaning. The more dependent clauses you add to a sentence, the more awkward it sounds and the more difficult it becomes to parse, whether in spoken or written English, so there are practical limits to how many levels of recursion your reader will tolerate.
Marcel Proust is infamous for some long sentences, which have been the subject of competitions and even artwork.