Without bumming everyone out, you don't need to be enjoying yourself to have an orgasm. Unwanted orgasms are sadly a thing. If I had to guess, I'd say if one of them is into it, it's happening, whatever the other one's subjective experience.

However, there's a bunch of anecdotal comments here suggesting conjoined twins are super close mentally, to the point of seeming psychic. Even if they have disagreements, chances are any difference in their daily mood would be minimal - shared hormonal spikes, shared bodily functions, shared daily experience... your body influences your mind just as your mind influences your body, just in this case there's two minds in the mix.

If one of them is stressing about their day, I would have thought the other almost certainly is too.