You don't say what makes/models of flashes you are using and that could make a difference. But...
If your second flash does not have an optical trigger (many/most studio flash units have one) then yes, you would need a third PW. If the flash you use has an optical trigger, and it can "see" the flash from the first flash then you don't need one.
My studio flashes all have optical triggers, but since I use softboxes with grids, barn doors, snoots, etc. to control and limit the light from a flash, in a great many cases the optical triggers don't work for me. The PW's always work.
Also, in case you don't know about it, Sekonic has PW-compatible radio modules for many of their flash meters, and that's something I use all the time. It makes it really easy to measure the light and set the flashes (I work mostly in fully manual modes).
The plug on the PW2+ is a standard 1/8" mono. If the 2 flashes are reasonably close you can plug a 2 - 1 splitter into a PW and run 1/8" mono extension cables to both flashes. The splitter is $5 or so at radio shack and the mono cables are $5-10 depending upon length.
I work with 2 Canon SpeedLight 580 EX
I don't have the big studio gear. it'll come some day.
So If I did understand well, I don't need an extra PW, since the first flash will trigger the second one? right?
the flashes won't be very far from each other: 6 to 8 feet
jean-yves
No, you didn't understand well. The 580 EX does not have an optical trigger and you will need some way to get the second flash to fire... a third PW would probably be the simplest and most reliable solution but the most costly.
If your first flash is off-camera and being triggered by a PW receiver (with a PW transmitter on the camera) the second flash won't fire simply because the first one does. You could buy something like a Wein optical "peanut" which would trigger the second flash when the first one fires. You could also try the wire splitter suggestion given earlier, but I'm guessing here that a wired solution isn't what you are looking for.
I'm really a fan of Pocket Wizards and that's the solution I would use. That way you don't have to worry about any optical trigger being blocked by a softbox, barn doors, furniture, etc. If you can afford it, go the PW route. It works.
Cheers!
(P.S. if you check my website you can get my phone number. Feel free to give me a call in the morning if you want. I see we're both in Montreal.)
The 580 EX does not have an optical trigger and you will need some way to get the second flash to fire... a third PW would probably be the simplest and most reliable solution but the most costly.