My husband (LPR) would like both of his parents to visit us for about 2-3 weeks this summer. My father-in-law has a crew-man's visa in his passport (he works on cargo ships, and can enter the US at various ports).

So they are getting ready to make an appointment at the embassy for an interview for the B2 tourist visa. Do you think it would be better if they went on separate trips, or together to the embassy? Do you think father-in-law having the crew-man's visa will help him prove he won't overstay the B2? (he had numerous entries/departures from the US in past yrs.)

I hear of stories that they don't let families travel together. I worry they'll suspect immigrant intent, with both parents traveling together, and given that their son is married in the US. (they have work contracts to tie them home, and land, but not sure this is enough)

What do you guys think? If anyone has stories, please share

I don't have experience with that yet, but it is one of my concerns for the future too.
Since his parents have property and jobs in Poland should be helpful same as Your father-in-law good record of entering and leaving US on time.
About going to embassy together or separately, seems to me more like lottery and it is hard to say what is better.
I think I would tell my parents go together and just being straight they want to visit their kid for 2-3 weeks and have them bring some proofs of their jobs, property, even someting proving they have other kids, grandkids(if they do:) in Poland so they have plenty reasons to go back to their country.
I wish You good luck

it will actually depend on the CO conducting the interview. i had my parents (both retired senior citizens) apply for their tourist visa a week before my CR1 interview in the US Embassy in Manila. when asked about the status of my spousal petition, they told the CO of my interview. they were given a 10 year multiple entry visa. my parents said a number of applicants from their batch were approved including two nannies who were going to travel with their employers.
since your in laws are still working and they have properties in their home country, they have everything going for them in getting a B2 approval. Goodluck!