Well just realized that getting a 2nd flash doesnt really make sense in terms of lighting advantages as 1 will stay on camera, (and I have built in for that) only advantage it gives is ability of high speed sync.
So I would have to buy 2 extra flashes for a total of 3 to give me extra advantages of composing. Need quiet bit of money for that....
Buying one extra flash allows you to do what you set out to do with only one flash anyway. Expensive, but it gets the job you were trying to do done. Buying a second (third) flash allows you to expand your lighting possibilities. You only need one 550/580/580II unit. You can use cheaper units as your slaves. The 420/430/430II will work fine as slaves, and are much cheaper. You can, I believe, use a 270II for backgrounds. To my understanding it is an all-or-nothing firing, though. You'd have to adjust the output manually, if that is even possible.
I have a total of about $700 in 2 550EX units and a 430EX2.