(If you can add an example, I would be more confident in my interpretation of the question.)
I would always use plural in possessions. For instance,
- "My friends' cars got towed last night." or
- "The cars of my friends got towed last night."
Clearly, the second sentence doesn't sound very natural, and both are a bit ambiguous regarding how many cars, or how many friends. It could be better written as:
- "My friends' cars all got towed." or
- "Some of my friends' cars got towed."
The second sentence is still ambiguous, and I would end up rephrasing it to make it clearer by saying:
- "Some of my friends had their cars towed (last night)." or
- "My friends had some of their cars towed (last night)."
Keeping the noun(s) and possession(s) next to each other in the sentence makes it much much more difficult. Either rephrasing the entire sentence, or using the "possessions of nouns" are better options, depending on context. For instance, the "of" form would work well in this case:
"The mayors of all neighboring towns are accomplished locals."
Note that if each noun has only one of the possessions, which is the case with mayors (one mayor per town), then the meaning would still be quite clear and unambiguous if we wrote, "All the neighboring towns' mayors ..."