How comfortable are you with using the flash off-camera? You have a radio trigger or a long cable or some other way to trigger the off-camera flash?
A common method is to use the in-camera settings to expose for the background, and use the flash to expose the subject. Your flash isn't going to light up a distant background. Conceptually, treat it like two separate photos that you're taking at the same time.
An alternate method - and the one I usually go to - is to use camera settings to eliminate most/all ambient light, and use your flash(es) to light the subject. The benefits of this are that you don't have to look for "good" ambient light or streetlights; you don't have to worry about the color difference between the ambient lights and the flashes. The downside is that you might not get an interesting environmental portrait like the one Ash posted above.
If you can get your hands on that second flash, do so. It doesn't have to be an expensive one; my workhorse flashes are two $70 Yongnuo YN-560s. As long as you can adjust the power level and zoom, and have some way to trigger it, it's fine.
I prefer to NOT use ETTL for off-camera flash work. If you have the time to set up your flashes and do a few test shots, set the power manually. ETTL will adjust the flash power based on what the camera thinks it sees in the scene, which you usually DON'T want - especially for moody and dark nighttime environmental portraits.
If you have two flashes, a tried-n-true reliable setup for moody and dramatic face, bust, and 3/4 portraits is the following:
Put your first light (main light or key light) in an umbrella or softbox a little above and to the side of the camera axis, and put the second light (kicker light or rim light) directly opposite that light, either bare or with a modifier. The main light creates your portrait lighting, and the kicker light gives you a separation from the background and adds visual interest.
I shot this with two cheap Speedlights, main light was in an inexpensive reflective umbrella and kicker light was bare. For the second split-light shot, I moved the main light pretty far off to the side.
This was in a kinda-empty area of my hotel at Metrocon. The background was lit by spill from the main light. I didn't do any wide shots because the rest of the surroundings were pretty ugly, convention carpet and all.