2 Flashes for Portraits
Personally what I would do is this:
Put your camera in Manual mode. Set one flash 45 degrees to your right (or left your choice) and above the subject's eye level. Diffuse the flash with a soft box, umbrella, or bouncing the flash (last is harder to control but involves no other equipment). The second flash behind your subject 180 degrees from the other flash. If your subject is away from the background point the flash back towards the subject for Rim Lighting. If the subject is close to the background, point the flash at the background to kill any shadows and if white you will get a little bit of Rim lighting. Again diffuse if you can (i think the YNs come with a little diffuser dome).
Next set your flash to proper exposure using the guide number, distance to subject, and aperture you have decided to use (I agree with the above that between f4-f8 is good). Then either set your on camera build in flash to 1 or 2 stops less than the exposure (fill flash and trigger), or use the wireless trigger and a reflector (white or silver) on the dark side of the subject's face.
That is a basic set up. You can experiment from there and adjust to suit the subject. Look into the types of lighting you want your subject to have (bold, narrow, Rembrandt) and adjust accordingly.
Check out this site for some references: http://www.diyphotography.net/taxonomy/term/172
For ambient light I would try to avoid any non-balanced indoor lighting. Either natural or white balanced lighting will be best. Otherwise you will want some color gels for your flashes.
Good luck and have fun