Casting on two-at-a-time sleeves or socks

I am a lazy knitter with serious second-sock (or sleeve) syndrome and an aversion to seaming things up, so I knit as much as possible in the round and two-at-a-time whenever I can. I have a question about casting on two-at-a-time magic-loop circular knitting.

I’ve done a couple of sets of sweater sleeves two-at-a-time on long circs (for magic loop) but seem to get screwed up during the cast-on. I know, theoretically, that I should cast on half the stitches for the first sleeve, then switch to the second ball of yarn, cast on all the stitches for the second sleeve (or sock), then go back to the first ball of yarn, and finish casting on for object #1… but I have gotten consistently screwed up in the joining of the stitches to knit in the round. I seem to have some kind of mental block.

Both times I had trouble when it came to getting the stitches joined and eventually (sort of) gave up, put one set of stitches on a different circ, joined them, and then transferred the two objects back onto the one long circ after a row or so when I coud better see what I was doing. Does anyone know… am I casting on correctly? When should the joining happen, or in what configuration should the stitches be when it does?

It took me several tries to get this down, it’s complicated! Here’s a pictoral guide for socks: