Layering fiber on the comb
Several manufacturers produce an assortment of wool hackles: big ones, small ones, round ones, single row, double row, more or less any arrangement you can think of. They are very nice. I don't have any of them. I don't have a stable place to mount a hackle where it won't fall off, fall over, or hackle some unrelated object like the sofa pillows. What I do have are hand-held wool combs. They look a lot like small hackles, and you can use them just the same. If you only occasionally want to blend fiber this way, you don't need a big hackle if you already have a set of single or double row combs. Just as in combing, more rows and closer set tines give better control of finer fibers. (You can do this with mounted combs, too.)
Hackles are like combs, they have tines and you put wool on them and pull wool off them. What is different is you may or may not do something in between. Like combs, they produce a smooth arrangement of parallel fibers and work best with fibers of uniform length. When you already have tops, either purchased or made, you can blend them together and still have that nice arrangement along with some interesting color effects. As an example, I'll go through producing a striped top using a wool comb as a hackle. They come in pairs but for this you only need one.
I have two colors of commercial top and I'm going to load the comb in two layers. Grab one and start running the end over the tines so the fiber catches. When you have as much as you want, do it again with the second color. You can put more on than you would normally for combing, because you are not going to transfer the fibers to a second comb. Since they don't have to move far, they are much less likely to tangle. Just make sure you can handle pulling that much fiber at once. I'd recommend you stick to not more than halfway up the tines.
Layering fiber on the comb
Now pull the fibers off together: gather the ends and tug a bit at a time to make a smooth bundle of fibers. Try to pull as evenly as possible across the whole width to keep the layers even and only pull about half a staple length at a time to maintain an even diameter. If you think this is just like pulling top while combing, that's because it is. It's actually easier to manage than a wide hackle because you don't have to work across the width of the tines.
Pulling the striped top
If you'd like the fibers more blended, make a lot of thinner layers. The easiest way to do this is to hold the two (or more) tops together and load them all on the comb at the same time.
Load the colors together for a more uniform blend
If you want an even more complete blend, you can comb the fibers to transfer them to the other comb and then pull the top. You can continue as long as you like and at this point it's exactly like combing from fleece.
When you are done, there will be short bits left in the comb. You can save those for another project if you like. Commercial tops vary in length even with the same kind of fibers. The white and blue in my example are both Merino but from different sources. The white is longer, so there is more blue left in the waste. Pay attention to what you are pulling off the combs to leave the shorter fibers behind. You could combine multiple lengths of top and draw them together to redistribute the fibers, but this will also blend the colors more.
Leftovers don't make nice top