Thanks
My triggers work the same as any other triggers do. So this question is not related to BL, but I would do it like this:
1. Put your animations into one group in timeline by naming them e.g. "group/Anim1" and "group/Anim2"
2. Now trigger timeline's "Play group/*" action from my orgasm trigger or any other.
Timeline automatically selects a random animation inside that group. You can even weight the animations (probability) in the Sequence tab. Keep in mind, that if you only have 2 looped animations timeline will cycle between them on trigger, it doesn't select the currently played animation again.