I have machines that have bobbins that are drop in, vertical front loaded and slant needle and twin needles work in all of them. They make a nice cover stitch and also good for sewing jean seams.

I have a very old Kenmore and it has two vertical thread holders on the top.
It has zig-zag capability and can accommodate as much as a 4 mm twin needle.
The two threads do fine going through all the same guides until right before
the needle-threading step. My machine has 2 metal loops just above the needle.
I use the twin needle primarily for stained glass quilt blocks. The 4 mm is perfectly sized to sew the 1/4" bias fabric that serves as the "leading" in the
stained glass.
Just experiment and you will find out whether your machine and you can work
with the 2 threads. It's a nice technique to have in your sewing armamentarium, but no need to stress about it. What's the worse that can happen - if the needle breaks, you can buy others. If you get a big knot
I'll bet that's happened before. Be playful. - Judy

I broke my twin needle playing around to see what it does. Now I need to get a new one. Not sure what I'm going to do with it, but one came with my machine, so I figured I could use it.

If it was included in your machine accessories, do yourself a favor and take the time to go through your manual. Mine has a section on using all the accessories. It would depend on what your manual has to say. Kind of like the laws in one state may or may not be the same in the neighboring state. LOL

I use my twin needle for decorative type stitching. I may be wrong but if it can zig-zag it probably can use a twin needle. I use my two threads on my two thread holders, thread each thread through the tension disks, an then thread each color through the eye of one of the needles. You get a really nice effect. I also use a triple needle for this sort of thing as well. You can do really interesting stuff with a wing needle as well. If you're going to play around, might as well try everything you can think of.

Make sure you walk your machine through the stitching pattern before taking off. You don't want to try stitching something wider than your machine allows!

Twin needles are very helpful if you do alot of heirloom or decorative sewing. They are really fun to use.

Twin needles can be used on top loading bobbin machines. I do it frequently.

Hi, I have a Brother CS6000i as well. The area where you drop the bobbin into is actually right in front where the needle is located. On top of the machine you will see like a peg where a spool of thread goes... and you should see what looks like a circular hole on top of the machine, that is where you insert another "peg" type plastic thingy (they give you that as well) it is for a second spool of thread to be used with the TWIN needle. I hope this helps.

I bought a twin needle for my Bernina. It makes the prettiest pin tucks on batiste material. Or really on any type material. The points are close together, so the bobbin thread pulls a small pucker to make it look like tucks. I love mine.

So what about using a twin needle to stitch in the ditch on a lap quilt???