The Sex Lives of Siamese Twins is extreme, and I'm not sure if Welsh takes it just too far. Lucy doesn't have a single redeeming feature, she is offensive towards her clients, shallow and violent. I don't think Lucy or Lena are particularly believable characters, I can't imagine anyone reacting the way they do, but as long as you can suspend belief they are interesting, it's like a car crash, horrible but difficult to look away.
Lucy is the stronger character of the two, and I wish that more time had been but into Lena's development. The blurb describes Lena as 'manipulative and needy', I can see the needy part, but I didn't really see her as manipulative. She has been taken advantage of by other people, first by her mother and then by her boyfriend. Lucy's behaviour towards her is abhorrent, but it could be argued it did her favour, but it is also questionable if the ends justify the means. The dubious morality of the novel is most fascinating aspect.
I wasn't sure about the subplot - conjoined twins who are contemplating separation surgery because one of the twins has a boyfriend that the other doesn't approve of, I didn't see the relevance to the main story. Kind of feel like Welsh had the idea of a story about conjoined twins, realised he couldn't spin it out into a full length novel, but didn't want to abandon the idea entirely, so he crowbarred it in here. Maybe there was some parallels I missed here? I suppose Lucy and Lena have a damaging, parasitic relationship, the same as the twins.
The Sex Lives of the Siamese Twins reminded me in some ways of Lionel Shriver's Big Brother, because of the same visceral hatred directed to overweight people, and the control and obsessiveness of calorie counting. It doesn't make comfortable reading, but it is effective. Disgust is a powerful emotion, and this topical subject matter is emotive. Both Welsh and Shriver are vicious writers, but I can't fault their talent.
I found the ending of The Sex Lives of the Siamese Twins to be slightly disappointing, to me it felt too neat, convenient and a little bit rushed. Though in fairness I'm not sure how else Welsh could have finished it, he sort of wrote himself into a corner.
If you are a fan on Welsh this novel is a fairly decent addition to his bibliography, it's not going to become a modern classic like Trainspotting but I would say I preferred it to Filth. You've got to admire Welsh for pushing boundaries, and The Sex Lives of Siamese Twins certainly does that.