They both got their bodies changed through contact with the Otherside, which is basically the Zone from Stalker but with more urban legends.
The black haired girl has a corrupted eye which can perceive the true form of things and change things according to her perception. The blonde girl has a translucent hand which can touch the true form of things and unravel or change it to her desires. Their powers are pretty esoteric and they don't understand them well either.
In the light novel, their relationship became romantic and they had sex for the first time. They were previously afraid to use their powers on each other, but when they had sex, they unravelled each other with their eye and hand, merged into one being and then put themselves back together. That's why they says that they did it normally this time, because sex for them is usually stranger, due to being partly supernatural. It's also why the blonde girl talks about the black haired girl watching carefully, since just being observed with the eye changes her body and probably enhances the pleasure.
This is the only ToraSora doujin I've seen but it was a good one. Very cute wholesome yuri that really brought out their characters. I know it is a lot to ask but I hope more Otherside Picnic doujins are drawn. I am particularly fond of the tiny white haired researcher girl, Kozakura. I hope to get to see can art of her as I love her character concept.
Well done and thank you tonall involved in bringing this translated work to sad panda.
Art 7. Plot 7. Sexiness 7. Translation 6. True to character 8. All out of 10.