My Rogue Trader's idea for shipboard security is in times of emergency, to outfit the crew with various equipment, the most important piecies being an adamantium combat shield (Into the Storm, for extra armor), a Mervoch Pattern Assault Lasgun (Into the Storm, for autofire), and recoil gloves (to offset the penalty for one-handed wielding).

My question is whether shield and lasgun combo counts as duel-wielding and therefore gets another -20 penalty. Shields are listed as melee weapons, but they're mostly being used here for blocking. So penalty or no penalty?

Try perhaps the tarsus-pattern Suppression Shield, Into the Storm, p123. With them, you can fire one basic class weapon with one hand without penalty...
If you prefer, just ask your DM to find a normal combat shield with the same kind of lock grips, three step rarer and losing the Primitive Quality.

I would say no since a shield is mainly used to defend at range. The only way it would fall under the duel-wielding is if they were in melee shield bashing away while firing a pistol.

As Fabian said, it is not dual wielding to hold a shield while shooting someone unless you want to hit him with the shield in addition to shooting him.

But by that logic the penalty would also never apply when you have two weapons, as long as you only shoot/hack with one?

Thinking about it, Dual-Wielding may mean wielding two things simultaneously (with an emphasis on "wielding" instead of just "carrying"), regardless of their nature. In the case of a shield this can easily be justified as obstructing the BS Test because the character has to block incoming attacks whilst trying to discharge his firearm at the same time - otherwise he'd loose the AP bonus the shield confers.

You could maybe propose a houserule that has the DW modifier not apply when the bearer "lowers his guard" for the round, loosing the AP bonus from the shield but being allowed to use that basic weapon without penalty. Basically making it a special stance.

Correct, you only get the two-weapon fighting penalty if your attacking with both weapons. If your simply holding the shield and shooting the gun downfield, the only thing the shield might cause is the one-handed penalty, but as Mordechai said there are shield that negate that.

Lynata said:

True but, unless they have ambidextrous, the weapon in their secondary hand has a -20 penalty for attacks. But not for parry (page 246).

Bilateralrope said:

Mmh, good point - though I could still see that transfer to this situation as "no penalty for shield parry* but penalty for lasgun and shield attacks".

(*: -some- shields have you roll to parry)

Isn't this clarified somewhere?

Page 246 of the Rogue Trader core rulebook is where you should look for details on two weapon fighting, though rules for fighting with both weapons without the TWW talent exist only in DH errata. Basically (where two modifiers are given, the main hand comes first):

- Parrying has no penalty, regardless of the hand you use for it.

- Attacking with the off hand is -20. It doesn't matter if your other hand has anything in it, or even if the other hand remains attached.

- Attacking with the main hand, no penalty. Again, the contents and location of the other hand are unimportant.

- Attacking with both: -20/-40 (from DH errata)

- Attacking with both when you have ambidextrous: -10/-30 (from DH errata)

- Attacking with both when you have the TWW* talent: -20 to both

- Attacking with TWW + ambidextrous: -10 to both.

*If mixing melee and ranged weapon, you need both TWW talents.

Note that this is before any modifiers provided by the weapon, and that shields get a further -10 to attack from the defensive quality.

By sheer coincidence, I just stumbled over the TWW rules in DH (p.197), which specifically state that shields count like weapons. It is of note that the RT TTW rules seem to have been copy-pasted from DH (at least it looks like the exact same wording), but the shield part is missing there.

Make of that what you will.

Lynata said:

This is the line from RT:

If you're weilding a melee weapon ... you may make a parry ... as normal with this weapon. ... This Weapon Skill Test is not an attack, and therefore it does not suffer the standard penalty for attacks made using a secondary hand

The DH rules omit that last 'not'. The copy-paste nature of a lot of the RT rules tell me that this is a typo, though one that didn't make it into the DH errata.

So parrying doesn't get the secondary-hand penalty if held in the secondary hand. But attacking with the same weapon does. It doesn't matter if the weapon is a shield or a hammer, both get treated the same by the off-hand penalty.

So to answer the OPs question: As long as the crew are holding the lasgun in their primary hand and don't attack with the shield, the dual-wielding penalty doesn't apply. However, a lasgun is still a basic weapon and you can't fire basic weapon when you are in melee, even if you are holding it in one hand.