Alright, I think I'm going for the 18 volt battery mod for my EMG 85/81 set. A few questions:

Do you simply put 2 9 volt batteries in there?
Of course it gives the pickups some extra power, but do the 2 batteries last longer than a single one?
1. Pretty much, just in series.
2. No

Hope that helps.
“Ignorance more frequently begets confidence than does knowledge.”
Charles Darwin
no extra power, just extra headroom (both my EMG loaded guitars are 18v modded). as for batter life, not if you do it properly. if you wire them in parallel, you will get extra battery life but no discernible tonal benefit.