I'm happy to see that single-headed drums have been mostly relegated to the used equipment ads, and that double-headed drums have regained their spot at the top of the food chain. But with more membranes comes more responsibly. Tuning a two-headed drum means tuning two heads and getting them to play nicely together.
Before we delve into the methods of tuning bottom (or 'reso') heads, we need to understand a bit of the theory and mechanics behind them. First, bottom heads trap sound inside the drum, and when we hit the top head, some of the sound bounces off the bottom head and back into the drum. This interaction brings more of the drum shell into the picture. The bottom head also influences tone, resonance, decay, and even stick response.
So let’s look at the three basic types of bottom heads: snare, bass and toms.
Snare
The thinness and tightness of the bottom snare head controls tone and pitch and also determines how quickly the snares respond. Tune the snare head low and your snares will respond slowly. This gives the drum a more "throaty" sound. Tightening the head yields more of the shell sound, improves stick feel, and allows the snares to respond more quickly. I keep my snare head “as tight as a board”. Anything less and the snare drum, to me, sounds flat and sluggish, and it likely won’t project well.
Bass Drum
These days the majority of bass drums have a fair amount of damping, often with a hole in the front head. Still, having even 80-90% of a front head will give you more resonance, more boom, and more beater response. That's important for feel as well as sound. The audience can feel a boom, whereas a sub-sonic 'blat' will go almost unnoticed. Drummers who want a full, round sound will keep both heads intact and cut back on the damping. They also tune the drum higher … in the 'boom' territory.
Toms
The old rule of thumb was, tune the top head for feel and the bottom head for tone. It works, although you'll end up with a fairly high “jazz” pitch. A better starting point is to tune both heads to the same pitch. I like to go for as much resonance and possible, which is usually found around the middle of a drum's range. From there I tweak the bottom head to fine tune. Tightening the head will reduce resonance and sustain, and usually increases overtones. Loosening the head can add resonance, sustain and a sense of fullness.
Take some time to experiment with those forgotten heads to make sure you get the most from the top heads.