I don't think there is a rule prescribing you should use the simple past after a present perfect or not.
However, the normal rules apply for the second (part of your) sentence in your examples.
If you take the second parts of your examples and make them independent:
Some people's accent was much thicker than mine.
I was at a conference together with a Japanese guy at the time.
You will notice that the meaning of the sentences changes if you would use a present perfect.
Some people's accents have been much thicker than mine.
This could be a correct phrase, I guess, if you are talking about the people you have met in your life. It is not a construction you will encounter often though. As having an accent is a continuous property, and it certainly was an ongoing thing at the time you met them, I see no reason not to use the simple past.
The same goes more or less for your Japanese man:
I have been to a congress with that guy.
Is a perfectly acceptable phrase, but it does not convey the same meaning. If you say you were at a congress at the time you met the man, simply use the simple past again.
