From Lexicanum

A twin-linked weapon is effectively two weapons of the same type positioned next to each other firing in tandem, therefore increasing the chances that the selected target will be hit.

The majority of twin-linked weapons are heavy weapons mounted on vehicles. Some of these include:

• Twin-linked Heavy Bolters
• Twin-linked Autocannons
• Twin-linked Lascannons

Some lighter weapons can be twin-linked, such as the twin-linked Boltgun, or Storm Bolter.

Sources

• Warhammer 40,000: Wargear