Only a guess, but I would say widows are territorial and one will die.
If you put a spider in there which isn't a widow, it will be lunch. I did have one other variety of spider that was pretty smart though, it was able to live in the same jar as the widow for about a month without getting eaten. It was very careful not to touch any webbing it didn't create. Then one day I put an ant in the jar, and when the other spider ran out and attacked the ant, it made the mistake of touching the widow's web..that was it's last meal.