First, the cover has a purple alien riding a motorbike down a road at night. As I recall it's sort of squid like, and wearing biker clothes.
The two stories are:
Three friends discover a device that lets them go to alternate universes, in the lab of a recently deceased parent. The three friends, two guys and a girl, get separated, and during the separation one of them is trained in a world were he learns to control his fear and do many things he thought impossible. Basically normal fat guy becomes bad ass and eventually saves the girl whom he secretly likes.
A guy on a dark road gets a flat tire and tries to wave down an approaching motorbike. This leads to an alien encounter when the rider is a girl working for a space law enforcement agency. While pursuing the bad guy the girl and the guy switch bodies and this affects their thinking. Girl starts to get horny, jealous, protective, the boy is less horny, more clear headed, and trying to solve the problems. It ends back on the same road at night.
I'm pretty sure I read this between 2000 and 2010.
I'd love to find the book again, or get the stories.