Math corrections.
Pyrotechnical
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$$\begin{array}{|c|c|} \hline & lv2 & lv5 & lv9 & lv13 & lv16 & lv17 & lv20\\ \hline 2x\ light& 15.65 & 25.43 & 29.36 & 30.28 & 33.28 & 34.21 & 38.21 \\ \hline g.sword & 13.34 & 27.30 & 29.30 & 29.95 & 31.95 & 32.60 & 36.60\\ \hline g.axe & 13.13 & 26.21 & 29.41 & 28.62 & 32.62 & 33.82 & 37.82\\ \hline \end{array}$$$$\begin{array}{|c|c|} \hline & lv2 & lv5 & lv9 & lv13 & lv16 & lv17 & lv20\\ \hline 2x\ light& 15.65 & 25.43 & 29.36 & 30.28 & 33.28 & 34.21 & 38.21 \\ \hline g.sword & 13.34 & 27.30 & 29.30 & 29.95 & 31.95 & 32.60 & 36.60\\ \hline g.axe & 13.13 & 26.21 & 29.41 & 30.62 & 32.62 & 33.82 & 37.82\\ \hline \end{array}$$

It looks like the only notable turn away from dual wielding occurs at level 5, before Rage damage increases at level 9 cause dual wielding to pretty much run away with things. I found it also interesting that the great axe begins to outshine the great sword fairly early on, which is contrary to conventional wisdom and it manages to pull ahead dual-wielding for a bit at 13th level.

• First attack: 1d12+6=121d12+7=13.5 dmg
• Second attack: 1d12+6=121d12+7=13.5 dmg
• Crit chance for 1 attack: 18.55%; therefore additional damage is 3.62 DPR
• Overall DPR: 2830.62 dmg
• First Attack: 1d6+10=13.5 dmg
• Second Attack: 1d6+10=13.5 dmg
• Third Attack: 1d6+8=71d6+4=7.5 dmg
• Crit chance for 3 attacks: 26.50%; therefore additional damage is 3.71 DPR
• Overall DPR: 38.21 dmg
Summary table.
Pyrotechnical
• 30.6k
• 20
• 111
• 211

$$\begin{array}{|c|c|} \hline & lv2 & lv5 & lv9 & lv13 & lv16 & lv17 & lv20\\ \hline 2x\ light& 15.65 & 25.43 & 29.36 & 30.28 & 33.28 & 34.21 & 38.21 \\ \hline g.sword & 13.34 & 27.30 & 29.30 & 27.95 & 31.95 & 32.60 & 36.60\\ \hline g.axe & 13.13 & 26.21 & 29.41 & 28.62 & 32.62 & 33.82 & 37.82\\ \hline \end{array}$$$$\begin{array}{|c|c|} \hline & lv2 & lv5 & lv9 & lv13 & lv16 & lv17 & lv20\\ \hline 2x\ light& 15.65 & 25.43 & 29.36 & 30.28 & 33.28 & 34.21 & 38.21 \\ \hline g.sword & 13.34 & 27.30 & 29.30 & 29.95 & 31.95 & 32.60 & 36.60\\ \hline g.axe & 13.13 & 26.21 & 29.41 & 28.62 & 32.62 & 33.82 & 37.82\\ \hline \end{array}$$

It looks like the only notable turn away from dual wielding occurs at level 5, before Rage damage increases at level 9 cause dual wielding to pretty much run away with things. I found it also interesting that the great axe begins to outshine the great sword fairly early on, which is contrary to conventional wisdom.

• First attack: 2d6+6=132d6+7=14.0 dmg
• Second attack: 2d6+6=132d6+7=14.0 dmg
• Crit chance for 2 attacks: 18.55%; therefore additional damage is 1.95 DPR
• Overall DPR: 2729.95 dmg
• First Attack: 1d6+8=11.5 dmg
• Second Attack: 1d6+8=11.5 dmg
• Third Attack: 1d6+8=71d6+4=7.5 dmg
• Crit chance for 3 attacks: 26.50%; therefore additional damage is 3.71 DPR
• Overall DPR: 34.21 dmg
Summary table.
Pyrotechnical
• 30.6k
• 20
• 111
• 211

tl;dr table:

$$\begin{array}{|c|c|} \hline & lv2 & lv5 & lv9 & lv13 & lv16 & lv17 & lv20\\ \hline 2x\ light& 15.65 & 25.43 & 29.36 & 30.28 & 33.28 & 34.21 & 38.21 \\ \hline g.sword & 13.34 & 27.30 & 29.30 & 27.95 & 31.95 & 32.60 & 36.60\\ \hline g.axe & 13.13 & 26.21 & 29.41 & 28.62 & 32.62 & 33.82 & 37.82\\ \hline \end{array}$$

It looks like the only notable turn away from dual wielding occurs at level 5, before Rage damage increases at level 9 cause dual wielding to pretty much run away with things.

Pyrotechnical
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• 20
• 111
• 211