I’m knitting a bag and there is a row with the following instructions.
They have me a bit confused. I know how to k2tog and k2togtbl. I’m not sure about in the same stitch. Won’t that be quite a bit of tension on the yarn? And I am really confused about the sl2 - cluster 2tog made. Isn’t cluster a crochet term?
Any help would be greatly appreciated.

“K2tog. Do not slip stitches off needle. K2togtbl into the same
Sts. Slip 2 st off needle - cluster 2tog made.”

The directions are telling you how to make the cluster and perhaps further along in the pattern that series of sts will just be called a cluster 2tog. Keep the working yarn a little looser than usual and you should be able to do the k2tog, k2togtbl. If you tighten up on the yarn, yes, it will become very difficult with too much tension on the working yarn.

This is where the use of nickel-plated, pointy tipped needles will help immensely.
Blunt bamboo needles are always trickier and harder to work with for this sort of maneuver.