Sometimes you need to work directional decreases on both sides of your knitting and this often involves P2tog through the back of the loops which is a rather awkward stitch to work. So let me guide you through an alternative way to work this stitch which I think is much easier on the hands and the final stitch looks exactly the same as a P2togtbl but requires less acrobatic hand maneuvers. There is also a video right at the very end of this tutorial.
Step 1: insert your right hand needle into the next 2 stitches just as you would when doing a P2tog:
slide these 2 stitches on to the right hand needle.
Step 2: bring the left needle up and under these two stitches and slide them back on to the left needle:
the stitches have now been twisted.
Step 3: Now purl these 2 stitches together just as your would a regular P2tog:
As you can see on the right side you now have a left leaning decrease that looks just the same as P2togtbl:
You can also view this tutorial on YouTube: