A few months ago, I purchased a Rivera Rock Crusher amplifier attenuator. I love the little booger, and it gives me so much control over volume and preamp v. power amp mix. Anyway, I was messing around with it one day, and decided to plug my Marshall 4x12 cabinet into one of the speaker outputs, while plugging my little Seismic 2x12 into the other speaker output. The attenuator was set to operate at 16 ohms, the cabs were both 16 ohms, and I was using the 16 ohm output on my Mesa. The sound was incredible. All of the sudden, the stereo signal scooped out a bunch of ugly mids, gave the sound a sparkling top end, and overall, made it sound much, much more aggressive. At any rate, the sound was DRASTICALLY different. So different in fact, that I can't even stand to listen to my amp running mono into my Marshall alone. So, my question is this: did splitting the amplifier signal with the attenuator change the sonic character of the amplifier itself (assuming that maybe the change in load had something to do with it), or is it just playing tricks on my ears? Or, perhaps, both? Is there a danger in running an amplifier this way?
Speakers are probably one of if not the single most important factor in an amp's tone.
The Marshall (assuming it's not an MG or FX cab) will probably have Celestions, most of which have a fairly pronounced mid-presence. I can't find what speakers are in the Seismic.
Here's a demo I did a while ago:

If you want the combined tone more conveniently, you could put the speakers from the Seismic into the Marshall cab alongside 2 of the stock ones.

On the subject of the impedance thing, generally the way it works is fairly simple;
1 x 16Ω = 16Ω
1 x 8Ω = 8Ω
2 x 16Ω = 8Ω
2 x 8Ω = 4Ω

For two different impedences, just multiply the 2 values and divide by the total (i.e 8Ω x 16Ω = 128, 8+16=24, 128 / 24 = 5.33Ω)

Lastly, you should always match the output to the cab, or go above it (in terms of impedance). What I mean by that is if you're using the 16Ω output, you should plug something with a value equal or greater than 16Ω into it (or risk damaging the amp and/or the cab). If the amp is set to 8Ω, as long as it's equal to or above 8Ω, you're good.

Hopefully some of that helps!

(if anyone want's to correct me on any of that, feel free and I'll edit this with more accurate info )