If two family members or friends are crucified together, they should be placed so that they can see one another, but not side-by-side. Each one can watch the other's torment, but they can't readily offer encouragement to one another.
Ideally, the one crucified first should be the one who's more likely to break down and cry and plead. Otherwise, the first might try to set an example of courage for the other, which would be unsatisfactory for the spectators.
Also, I assume that when the cross is first raised, the crucified is totally occupied with the pain of hanging from the nails, and has no attention to spare for anything else. If the second sibling/friend is crucified immediately after the first, then the first won't really notice. If we allow a little time to elapse before crucifying the second, then the first will be more likely to watch, and will suffer the psychological torment of watching a loved one's humiliation and torture.
I've got a scenario in which two siblings are crucified as part of a group of five. The younger sibling is the first crucified, and the older one goes fourth. This puts them 25–30 feet (8–10 m) apart, too far to murmur encouragement to one another. The crucifixion of the older begins after the younger's been hanging from his cross for roughly 10 minutes, so the immediate shock of crucifixion is over and the younger can pay some attention to the stripping and nailing and raising of the older. Since the crucified are arranged along an arc, the two actually have clearer views of one another than if they on adjacent crosses.