Sure. Many double portraits have been shot using just window light and a reflector, or one artificial light and a reflector; it just depends on the look you're after.
On a basic level lighting a human face is an exercise in getting light past the brow and nose with the key light and into both eyes, while at the same time making the nose shadow light in tone and non-distracting. The problem is when fill gets moved to the side it gets further away from the nose than it would be over the camera, so the nose shadow become one of the darkest and most distracting shadows, in part due to the fact it sits between to brightly lit cheeks. When the fill source moves to the side back past the nose it will become shaded by the cheek and you will notice that the smile lines, corners of the mouth, side of the nose, and base of the nose get very dark shadows in areas neither the fill or key light reach.
The problem with reflectors indoors is finding a spot where is effective at filling all the shadows the camera sees (i.e. on the FRONT of the face) and being able to catch and reflect the key light. If you do use a reflector the bigger it is the more effective it will be an the further forward you will be able to place it.
To better catch the light of the key light you'd be better off using an umbrella on the key light and feathering it forward. By feathering I mean aiming with the edge of the key light.
The first problem is getting a pose where the key light will be flattering on both faces at the same time...
Start with the guy facing the key light placed 45 degrees from the camera axis and also 45 degrees from his nose when looking at the camera. Then put the woman facing him as on the lower right. Her back will be towards the light and face in shadow if looking at him, but what you need to complete the pose is to turn her face back towards her shoulder to look into the camera. That will bring her face into the key light at the same angle as his and you'll get an identical pattern. Keep the the heads together - touching with no gap for married couples- and it will create "together" body language. The bigger the gap between heads the more difficult it is to get the same light on the faces and the less connected they will look.
Once you get the basic key light pattern set rotate the umbrella on the stand forward until it is completely in front of them. Then slowly bring it back onto the face. You will see the light is softer on the edges than the middle. What you want to wind up with is the shaft of the umbrella pointing past the front of the faces. In addition to putting softer light on the faces than the brighter middle it will allow you to place your reflector further forward relative the the faces. With the umbrella you will allow get the benefit of wrap around fill and fill from light bouncing off the ceiling and opposite wall. Setting up close to a light wall on the shadow side will help give you a wrap-around fill effect similar to the sky outdoors.
Once you get the "short" lighting pattern set on the face you can capture well lit oblique and even profile views by just walking around the faces with a camera. That's easier to do with one person...
Outdoors one of the best strategies is to keep the sun off the front of the subject entirely (i.e. back to the sun), expose for the background first, which will make the front of the subject quite dark, then light the front with the flash / reflector to balance.
The the front of the face and body will start out with just the fill from the sky. Any other source you add (flash / reflector) will actually be creating highlights over that fill (acting as front main/key light) so you need to keep it above eye level as you would any key light. The reason flash on camera looks "fake" is due mainly to the angle. Natural light comes from above the face (most of the time) and artificial light should too it you want it to look natural.
Also, in backlight the brow will still shade the face in shadows resulting in the shadows in the eyes being darker. After you add flash or reflector the eyes will still be slightly darker than the highlights on the cheeks. The solution is to bring along a 4-6' ladder. Stand on the ladder and have the subjects look up at up. The camera/face angle will be the same as it would at ground level, but by raising their chins the skylight will reach the eyes and cheeks equally eliminating the dark eye sockets.
