Originally posted by Unsinkable II I'm looking to having two off-camera flashes, at different power levels, playing nice with each other and the ambient light too, and, in combination, get a correct exposure.
You won't get that with PTTL. A PTTL flash (or flashes, but I don't think the PTTL system is designed for multiple flashes) will fire at the power it feels is needed to get a correct exposure. You won't get a simple way to control the light output of each flash short of filtering one and hoping the other will compensate.
What you really want is a simple manual setup. It may sound scary but it's actually much easier to get the results you want with manual flashes. Just get a wired or wireless trigger that does only that. Then switch your camera to manual, set your flashes at one given power each, and test. Then adjust as needed. Once you're set, you never have to change anything for that session, and chances are you will be able to use roughly the same settings in future venues.
As for the camera, here is what you ned to know:
Aperture is for flash illumination (wider = more flash-made light on your picture)
Speed is for ambiant (faster speed = less ambiant light, better for, say, dark backgrounds or exotic lighting schemes)
ISO influences both
Pretty straightforward. Try it!