Hello H R Ford,
I think you probably have this licked from reading the other replies, but here’s my advice for what it’s worth.
Use the cameras manual exposure mode, not Av, Tv, P or anything other than M(annual)
Set the shutter speed at the maximum flash sync speed for your camera or less. This is 1/200 for a Canon EOS 350D.
Use a flash meter to calculate the aperture you need and the ratio between the heads if you are using more than one. You cannot rely on the image preview for anything other than a rough idea. When you look at the flash meter reading for the main light, lets say f8, you would use this as your camera lens setting. You would then need to decide if you want any other light(s) the same, brighter or darker i.e. if you use a second light on a white background you would want to set its output so that the meter reading for this head is f11. In this way you would have a white background with a correctly exposed subject.
You can sync the flash head with a cable from one head to the camera and the second head set to slave (you would need the hot shoe adapter shown elsewhere to do this on a EOS 350D) or you could use an IR trigger, but these tend to be flaky even when one or more of the heads are fitted with an IR receiver. Best of all (and most expensive) is a radio trigger one on the camera to transmit and one on a head to receive with the other set to slave or even better still a receiver on each head (plus the camera transmitter of course!).
Ambient light should be kept to a minimum in the studio to avoid effecting the lighting effect you have created, studio flash is about controlling the quality as well as the quantity of light.
With flash only exposure shutter speed is not an issue as even children are slower than light. For some specialist shots such as the famous bullet passing through a light bulb or playing card a “high speed flash” is used. This is simply a flash head (or bulb) with a very short discharge time. For all domestic/normal applications any flash unit will fire faster than the fastest shutter speed setting your camera has for ambient light photography.
I hope this is of some interest to you.
ilikebowens