I just did some tests in arena mode. The creatures used were humans and all items were iron.
15 with 2-handed swords and shields Vs. 15 with short swords and shields
Result short swords win with 13 survivors13 with 2-handed swords and shields Vs. 13 with long swords and shields
Result long swords win with 11 survivors15 with 2-handed swords Vs. 15 with long swords
Result 2-handed swords win with 7 survivors15 with 2-handed swords Vs. 15 with short swords
Result 2-handed swords win with 9 survivorsThese results suggest 2-handed swords do get a debuff when wielded with other items.
Edit: other's results
By thvaz
15 twohanded sword humans Vs. 15 long sword with shield humans Result 3 two handed sword survivors
by Shinotsa
First test (No skill humans)
15 humans each with an iron two handed sword vs 15 humans each with an iron two handed sword and a shield.
Result: 12 survivors on the shieldless team.
Second test (Grandmaster fighter, dodger, swordsman and shield user for all combatants)
15 humans each with an iron two handed sword vs 15 humans each with an iron two handed sword and a shield
Result: 11 survivors on the shieldless team.
Reran the final test with featherwood shields and iron shields. Featherwood showed no change in results, iron ended with 14 shieldless, multigraspers alive.
edit: New tests
20 with 2-handed swords Vs. 20 with long swords and shields
Result long swords and shields win with 12 survivors15 with 2-handed swords Vs. 15 with short swords and shields
Result short swords and shields win with 12 survivors15 with 2-handed swords Vs. 15 with scimitars
Result scimitars win with 11 survivors15 with 2-handed swords and shields Vs. 15 with scimitars and shields
Result scimitars and shields win with 10 survivors15 with 2-handed swords Vs. 15 with scimitars and shields
Result 2-handed swords win with 6 survivors