MIHC,
Does "multi-pilot aircraft" not refer to mandatory multi-crew aircraft? Not simply a single-pilot aircraft that happens to have two crew on the day, such as a PA31?
This has always been pretty simple to me, if you're crewing an aircraft you have to log something, and co-pilot time is pretty much by definition time logged when the other categories don't apply (ie. PIC, ICUS, Dual). It's ridiculous to suggest you can't have two pilots crewing a PA31, required or not - and the second pilot must log something.