Other thing to watch out for is multi-tasking with 2 girls, can only think with one brain at a time myself so others might have the same problem.
I think it's a good point (no pun intended) and thanks for bringing it up.
It can be like having a different part of your brain stimulated. Periferal awareness. Like when you're chatting up a girl in a bar, looking at her and smiling at her, but her pal, who you can just see out of the corner of your eye, is somehow radiating the energy that's giving you a big erection.
The nice thing about a threesome when this happens is you don't have to be so disingenuous about it. One is stimulating your conscious brain and the other is stimulating the edge of consciousness, and it's horny as hell.
The danger for the newcomer to three or more-somes, as you point out, is that he feels confused about which one to focus on. I'm trying to analyse what I do but to get the benefit obviously you have to keep both girls occupied. If I'm having a full-on interaction with one (eg fucking her) then i want a different type of interaction going on with the other, even if it's only stroking her thigh (a tactile thing, and which can be very erotic, especially as the first one is satisfying the more mechanical 'pumping' need).
I demand interaction usually. If I'm stroking the second woman's thigh she has to be focussed on that and getting into it to some extent. The easiest starter can be fucking one and kissing /breast fondling-or-sucking the other. If the women are inexperienced, they can also get disorientated though.
My template for three and more-somes is swinging orgies or, to an extent, LMP (which is basically swinging re-jigged for male pleasure instead of women running the show as in swinging). Swinging can teach a certain awareness, which comes via respect for the wishes of others. A number of times at Machester's Cupids for instance, I'd poke my head into a room where an orgy was in progress. A woman being fucked might invite me to stick my cock in her mouth, and meanwhile another woman might be giving me the eye (across several bodies) with a sort of "us next" come-on look. With no requirement to look at the woman giving me a blow job (sideways on as she's fucked missionary), i can enjoy the flirtation.
This suggests another psychology of what is happening, and that is anticipation vs desire, or savouring your steak while also looking forward with mouth-watering expectation to the creme brûlée! (I seem to remember I read some good essays by Ross Jeffries on this subject many years ago btw. He is the only 'gamemaster' guy in the bunch of wannabees that actually understands anything about psychology.)