Flight lands on Mars - 1 man escapes - Human arm comes to life - 2 part novel

[qjmac]

i read this about 10 years ago, in paper back. It was new at the time - sold by register at borders. It was written in 2 parts, the idea you could start at either part and the story made since. Part 1 - team flies in separate ships to Mars. Two of the astronauts have sex on their ship - real sloppy. Being on mars effects one of them mentally - things go crazy and the narrator manages to escape. He does and his ship crashes - his arm is infescted. Part 2 - the arm has a mind of its own and infects humans and other zany situations in the desert I believe near Vegas.

I’ve googled the hell out of this - any help appreciated