I'm pretty sure they're the same thing. I never actually slip/slip/knit; I just go in the back of the loop of two stiches (thus, tbl). I used to knit combined style, and that's how knit stitches are done that way, so I recconginzed the yarn/needle configuration.
When you K2tog, the stitches slant from the lower left to the upper right. When you SSK, the stitches slant from the lower right to the upper left. So if, for example, you are doing decreases on both edges of a sweater or a mitten and you wanted it to look like this:
// \\ instead of // //
you'd use the different techniques on the different sides. They both decrease, but it looks nicer if you make the stitches lay the direction that they should.
Once again, I'm sure that I'm clear as mud, but trust me that it does make a difference.
I guess what I'm talking about (god knows I may have miss read the origional question of the thread) is the difference between knitting two stitches *as normal* (is in through the front of the loops on the needle, just as you would a single stitch in un-combinded knitting) for k2tog and knitting into the *back* of the loops for ssk (as in combinded knitting).
I'm completly aware of the difference; it's the technique I'm looking at.
I finally sorted out the way I ssk (by knitting through the back of the loop combined style without slipping at all) by ssk, then comparing the relative needle/loop position to combined-through-back-of-loop position.
The way I do it (through back of 2 loops) works great on all my stuff, including the ever delightful gusset decreases on socks.
I was knitting this dishcloth with a diamond eyelet pattern from a Leisure Arts book and it called for both K2tog and K2tog tbl. But I noticed the K2tog tbl wasn't a perfect mirror-image match for the K2tog because there was a twist. Well, considering Leisure Arts is notorious for "simplifying" patterns in the interest of space, I figured it took less space for them to put in a K2tog tbl diagram than an SSK diagram because it was obvious in the model photo that the knitter did *not* use K2tog tbl.
Anyway, forgive my rambling. The simple version is that K2tog tbl has a twist in it and SSK doesn't which makes SSK a better match to K2tog.