Saving time and money

I'll first point out that i wasnt sure if this was the right category, but ive put it here just incase.

I've decided i want to get the expansion packs for sins (yeah, i know, bit late for a sins fan). When i bought my copy of sins i installed it on two computers to play over lan. I want to get the expansion packs on bothe computers but i dont want to pay twice as much. if i was to buy them using impulse could i install them on either computer from my impulse account?

I would really appreciate the help. (im not so sure if i posed the question very well so just sing out if u need clarification).