What's the trick to hanging overhead light fixtures with only two hands?
Many flush ceiling fans have a built in solution in the form of a hook that hold the motor near the ceiling while you attach wires. Then you lift he fan off the hook and secure to the mounting bracket.

You can rig a similar hook if you can find a secure point on the lighting fixture as a grab point (either through or around). Use a wire coat hanger to make a double ended hook. Use at least two strands of the wire and twist ends tightly together for strength. One end of the double hook goes through or around your fixture grab point and the other hook end goes over the mounting strap attached to the ceiling box.

Support the unit (make sure that the hook mechanism seems strong enough to support the weight), attach wires, then lift unit and slip hook mechanism free. Attach mounting screws.