I can't seem to find an option for Two Weapon Wielder except for Tactical and Assault Marines. I would have thought this open to all marines. Have I overlooked it, or is it an elite advance only? If so, how much would GMs charge for it?
Elite advice for Two Weapon Wielder?
It's in the errata. Two Weapon Wielder (Ballistic) and Two Weapon Wielder (Melee) have been added to the General Space Marine Advances at Rank 7 for 800 xp apiece.
Would you mind pointing me to the errata?
For future reference, a Google search or accessing the "Support" section of the Deathwatch page would have gotten you there.
I allowed a Fleshtearer librarian PC in my own game to take a variant of the "No Quarter Given" deed on p83 of Rites of Battle, which gave two weapon wielder (melee) instead of counterattack. I thought it a reasonable swap considering how useful counterattack is.