Weapons are sometimes linked to fire together in order to increase their chances of scoring a hit through the crude expedient of blasting more shots at the target. To represent the fusillade of fire laid down by a twin-linked weapon you may re-roll the dice to hit if it misses. However, if the second roll is also a miss you can't re-roll the dice again.
Twin-Linked Weapons
Source: 3rd-A-Palooza (3.5) Rulebook v1.4, p. 16