I'm working on a part of a song where I need to strum the top two strings (B & E) together very fast. I'm wondering if there is some sort of a damping capo that I can put over the other four strings so I don't have to worry about accidentally hitting them . . .
I think the most important things that I have learned during my years fiddling with a Guitar is to desperately work on honing the accuracy of your picking instead of going at the strings like a madman (if you'll pardon the expression :D). I often used to play with a heavy hand; any time I wasn't soloing, i.e. picking individual strings, I was hammering more strings than I needed and muting those that were unneeded with a combination of left and right hand trickery. I blame this anti-technique on learning to play on an acoustic first, where whacking out G/C/D chord progressions tempts you to sin against your right hand accuracy. Being self-taught, I had no one to correct me as I'm sure a good guitar teacher would do.
At times, playing fully muted strings in addition to un-muted strings is a very useful technique. For some great examples, see Where the Streets have no Name by U2 and pretty much any guitar work by The Edge or musicians in that similar genre. Some blues players like Stevie Ray Vaughn, would mute strings and "rake" in on the starting phrase of a solo to give it a scratchy, soulful sound. So it's not bad to play muted strings, it's just important to do it intentionally.
In conclusion, to fix this I would encourage you to work on your right hand accuracy by doing picking drills, practicing string skipping, and improving the overall precision of your right hand instead of putting a Band-Aid on the issue with capos and muting tricks. Focus on only hitting the strings you need to play the phrase. Everything from textural guitar to speed metal relies heavily on right hand accuracy if you want your playing to sound clean.
You can use an elastic band of some sort (hair band etc) to dampen the strings.
Or you can have something made and attached al la this web page (not sure if these things are still around, but im sure you could get something made).
Or as silver light suggested; use your fretting hand or picking hand palm.
If you worried about feedback and/or string noise you might also think about getting a noise gate.
EDIT: It peaked my interest as to whether you can still get string dampeners; so I looked and found The Batten String Damper.
Greg Howe uses a scrunchie the same color as his guitar body. He pulls it above the nut when he needs it. He mostly uses it to fight ringing adjacent strings when he's doing tapping, but you can use it for other things.
You should develop pick accuracy. You should develop fretting-hand muting. But while you're doing that, you can use this to quiet stuff down some.
I can offer a little bit of advice, as I have been in this position with twelve strings a couple of times, playing it to sound like a mandolin by only using the top 2 strings.
If you don't want to use your right palm or your left hand fingers to dampen/mute the string, I have a couple of ideas for you.
If you have one of those Shubb type capos, I would suggest putting it on but only clamping across the lower 4 strings, leaving the top 2 exposed. You will need to unscrew the screw nearly all the way so that when you clamp down so that you do not damage the wood of the neck. Make sure the tightness is just enough to dampen the strings, and if the capo is a little loose, tighten the screw slightly. Be careful that the capo does not slip.
Another option would be to put some blue-tac on the string you want dampened half way between the nut and the first fret.
Hope this helps.