scythe of the cycle boosts only secondary skills:
As for using 2hand or 1hand+offhand, I am not sure how exactly damage formula goes, but you need to take into consideration that +dmg on offfhand adds on to weapon damage:
red = damage per second (calculation of average damage * attack speed)
blue = actual weapon damage, this is base for all calculations of skills and multipliers that use %WEAPON DAMAGE
So if you take a look on this example items:
You get something like this:
without any other multipliers (main stat, gems, other attack speed multipliers etc).
So, you get more damage using 2h, but you loose good deal of stats (area damage, crit chance, +skill, int/vit).
As for trang weapon, most of builds are using it for second part (red part under) of it's legendary property:
The Cursed Scythe rune for Grim Scythe now has a 100% chance to apply a curse and you deal 150–200% increased damage to cursed enemies.