One of my players wants to use his chain sword with 2 hands, does it provide any bonus? Can't find an answer in the core rulebook or forum

By RAW - no. He can change chainsword to eviscerator (big 2-handed chainsword).

In most cases, it's probably not an option. But IIRC for some weapons there were stats given for both 1h and 2h grip.

5 hours ago, TBeholder said:

But IIRC for some weapons there were stats given for both 1h and 2h grip.

Only if you use two-handed weapon with one hand.

(looking into Armory) there are changes for 2-hands use:

• Nemesis Force Halberd (in Daemon Hunter and DH2 ) has [Tearing]
• Double-edged "Mercy" chainsword (from Into the Storm ) - said to have a long haft so that it can be wielded both ways (bastard chainsword?) - does not have both variants specified, but in stats has 3 more damage than normal chainsword (and 1 more than chainaxe, while having less weight), which does correspond with average human SB. In which case, when wielded one-handed it would be just an overweight chainsword without [Balanced]. Why not.
• Cognatine Hammer (in Faith and Coin ) has +2 damage, it's a power weapon and already [Unwieldy]
• Power Mace (in BC ) / power mauls (in OW and DH2 ) have [Concussive(0)], as do Polix Heavy Khopesh (in Tome of Fate ) and Accursed Crozius (in Tome of Excess )
• Chainaxe (in Hammer of the Emperor ) has +2 Pen and [Unwieldy] instead of [Unbalanced],

That's all?

On 5/9/2017 at 9:14 PM, Jargal said:

Only if you use two-handed weapon with one hand.

This just depends on from which end you look. If there's no equivalent of Pistol vs. Basic vs. Heavy distinction, there's no equivalent of Handcannon vs. Carbine distinction either.

You could always go oldschool (DnD status) and allow an additional 50% strength bonus (rounded down) added on. In other words, if their SB is 4, the weapon will hit with 1d10+6 (1/2 of 4 is 2, so 4+2 = 6). That shouldn't be game breaking and it will prevent the PC from using anything else in the other hand.