Is there any voltage/power issues to deal with for running multiple flashes on cords?Modern cameras and flashes use low-voltage logic to trigger, so you're unlikely to run into any problems like that. Older flashes that use high-voltage could damage your camera regardless of using a cord, but using anything generally recommended for a modern Canon camera should be OK.
The only reason I say 'should' is that if you use two different kinds of flashes they might "pull up" the trigger to slightly different voltages. That wouldn't damage the camera, but could damage one of the flash units. Frankly though a flash would have to be very poorly designed for that to happen so the chances are slim. Personally I wouldn't worry about it, especially if your're planning on an EX-series for the second flash.
Edit - another thought, I wonder if the PC sync cable still 'fires' with an ETTL flash attached? That way you could have full control over one flash and use the other manually. I've never tried it myself but if nobody else knows can try it tomorrow on my 7D and let you know.