The idea of layering snares is to make one snare sound that has alot of weight behind it, by eq'ing them all the same you are being counter productive. Also, you musnt always eq snares in the same way, it all varies depending on what samples you use.
For instance, if i was to layer two snares together, i would first find one with a high end snap, and another with a punchy low mid sound to it. From there i would put a spectral analyzer on each and see where they peak. I would boost the high snappy snare where it peaks just slightly, and note that frequency. I would then go to the punchier snare, and cut quite heavily around the frequency that i noted from the higher snare.
From there i would group them together and add another eq, and mould to taste
by no means am i an expert so please correct me if this is wrong but in addition to mr fletch's post you should also try and envelope the snares (in a sampler) so you are only using the bit you need e.g.snappy attack and splashy release, once your happy withe the sound and processing bus/group your snares together and add light compression for a unified sound.
i tend to get three snares one for each range e.g. a low sounding snare mid sounding snare and a high sounding snare then mix and eq them together so they dont clash too hard otherwise your headroom will just die
i get 2 snares, one from a break which i use subtractive eqing, always sounds better on those old crisp breaks, then layer with one clean snare and eq and use an envelope if i want to tighten it up. I normally dont really eq a sampled snare though cause they've been processed before, sometimes i just use overdrive.
The key to a good snare is probably finding one that you dont need to eq or process. It just sounds good!! then layering a real snare gives it that real snare feel.