There is no standard in nomenclature. However, if your method is going to be acting on compound types, consider adopting a convention of using Get...And...() to indicate that there are two things going on. For example:
int GetPopulationAndMeanAge(out double meanAge)
{
// ...
meanAge = CalculateMeanAge();
return totalPopulation;
}
I think the better approach is to return a compound type instead. In a garbage collected language, there is really no excuse NOT to do this, except in cases where such a method is called, say, millions of times and instrumentation reveals that the GC isn't properly handling the load. In non-GC languages, it presents a minor issue in terms of making sure that it's clear who is responsible for cleaning up the memory when you're done.
Refactoring the previous into a compound type (C#):
public class PopulationStatistics {
int Population { get; set; }
double MeanAge { get; set; }
}
PopulationStatistics GetPopulationStatistics()
{
// ...
return new PopulationStatistics { Population = totalPopulation, MeanAge = CalculateMeanAge };
}