The "body swap" is in yet another manga titled 二人のヒミツ, pg 197. Probably the third tg manga named two people's secret - they come up with new titles. They don't get along but at night they share the same dream, and one night they have an erotic dream where he becomes his sister and masturbates. (Because he'd caught a glimpse of her naked in the shower earlier.) They both awaken aroused.
For the body swap one: They share the same dreams when they sleep, and one day the brother Rui accidentally sees his twin sister Yui naked in the bath. The dream they share that night then is Rui seeing himself as Yui and starting to masturbate. After they wake up, they admit they're both curious about each others bodies, and sit on the bed naked. Then of course, they decide to take it further and start making out, and eventually go all the way. The next day their friends comment on they seem to be getting along really well today -- did something good happen? Oh not much, just a dream we had last night...