@simplistic.jon
It is a totally viable build and it does have a lot of synergy.
To be honest, while there might be some difference in damage between 2H spear and 1H+Dagger, it isn’t too significant because your DPS is mostly decided by the skills that you use rather than the weapon you wear.
You can use the TOSBase items database to search for the possible weapons you could use through the game, but here are a few examples:
1H Spear + Dagger
2H Spear
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Elements
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Up to 317 raw damage against all targets.
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Catacombs Pike
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Up to 600 raw damage against Large Type targets and 412 against other targets.
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Lolopanther Pike
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Up to 680 raw damage against all targets.
As you can see, 1H Spear+Dagger deals more raw damage compared to the 2H counterpart. What is making the difference is the Dragoon attribute that increases the damage against Large enemies.
Is it a significant difference? Yes it is, but you also have Hexen Dropper on your build which provides you with a different burst type of skill to deal damage and under the right circumstances it could be more useful than what other Dragoon builds have to offer.
Not to even mention that the extra damage you are losing is also gained as utility. Unlike other Dragoon builds, you are able to use Iron Hook and Jolly Roger, which is part of the reason for choosing Corsair anyways.
I can’t confirm whether Venom’s poison effect works with Double Weapon Assault. My guess is that it does and if this is the case then that is another good benefit although not as reliable as the Dragoon attribute.
But you do have the Corsair’s attribute that increases your damage by 12.5% when attacking an enemy from behind, so that is another thing to take into consideration (although this requires proper documented testing to conclude how effective it is in practice),
The Dragoon attribute also only really works against Large enemies and bosses, so it doesn’t make any difference on something like PvP, where either weapon set (1H+Dagger or 2H Spear) pretty much deal the same amount of damage.
So, follow your dreams and don’t be too discouraged by not being able to use the 2H Attribute from Dragoon because Corsair+Dragoon is a solid build choice.