Secondary professions for an Assassin

Assassin/Warrior
Due to the high dagger attack speed, this combination gains a lot of adrenaline, so it's possible to use adrenaline skills to great affect. Skills such as Rush or "Fear Me!" are good examples.

Assassins do not have any attack speed enhancing stances, and can thus greatly benefit from Warrior skills such as Flurry.

Assassin/Ranger
Barrage damage can be improved with high critical strikes attributes, adding bonus damage to attacks, energy bonus, and effects from skills like Way of Perfection.

Assassin/Elementalist
The knockdown from Shock can drive an attack chain, though it is no longer as popular as other skills such as hexes to drive the attack chain. As always, the Conjure line of enchantments can help with damage dealing.

Assassin/Mesmer

 * Use Arcane Echo to chain Shadow Form.

Assassin/Monk
The Protection Prayers skill Mending Touch is a good spell to remove risky conditions.

Assassin/Necromancer
Curses hex spells can be used to drive many attack chains.

Assassin/Ritualist
In PvE, Splinter Weapon works well with Death Blossom.

Assassin/Paragon
This profession combination provides little benefit for an Assassin primary.

Assassin/Dervish
The Assassin Dervish can use Critical Strikes to greatly improve the Critical Hit rate with a scythe. Scythes, having a huge damage range compared to other weapons, provide the highest maximum damage value for critical hits.