The two shot
Simulator
One key, two faces
A key that flatters one face will usually do something unhelpful to the other: underlight it, or lay a nose shadow across it. Everything about lighting a two shot is the negotiation between those two faces.
The difference between them
How much brighter one face is than the other, in stops. Under half a stop reads as even; over two and the darker one looks like an accident.
Why distance evens them out
A lamp close to the pair is much nearer to one than the other, and inverse square does the rest. Pull it back and the ratio between the two distances shrinks, so both faces get almost the same light.
Cross keying
A unit on each subject from the opposite side, so each one’s key is the other’s backlight. It costs two lamps and buys an even, shaped result that survives both of them moving.
The nose shadow problem
A key wide enough to shape one face throws that face’s nose shadow across the cheek, and at a certain angle one subject’s head shadows the other entirely. Watch for it as the angle passes about seventy degrees.
Spacing is a lighting decision
How far apart the two stand is usually treated as blocking, but it decides your lighting problem. Close together and one lamp can do it; a metre apart and you are choosing between distance and a second unit.
Backlight does the separating
With two people against the same background, a backlight is what stops them merging into it and into each other. It is doing more work here than it does on a single.