Technically, there is no such thing as negative alpha. The lowest Alpha can go is 0 (fully trnasparent). So, both images always have positive alpha.
But, you can use a map value node to pass on or filter out values below min and above max, like a band-pass filter. For the values that are passed thru, you can scale them using a Colorramp node with a very narrow range set with the upper slider very down low, so that very little values input are output as 1.0 (white). Then compare that to the original using a multiply mix node, and wherever the band-pass map value output is 0, those values (color, Z or A) will be set to 0. Wherever the passed values are 1, the image is preserved.
You can also set the offset to your threshold, like -.5, and your factor to -1, and the MIN button on to 0. You can then add .5 using a math node. Then anything that come in will have .5 subtracted from it, turning negative. the factor of -1 will flip that to positive .5, and the MIN will pass it thru. Anything that WAS above .5 will be positive then flipped negative and thus filtered out. See the wiki on the Map Value node for more info.