What is the damage over time formula?
 

My best guess for damage formula for Primary Hand:
average damage = (weapon damage*2 + damagebonus)/2
damage over time = average damage / weapon delay

And offhand formula:
damage over time = (weapon damage/weapon delay)

Level 37 and damage bonus of 4

Primary Hand
Adamantite Club average damage = (15*2 + 4)/2 = 17
Adamantite Club damage over time = 17/25 = .68

Primary Hand
Stave of Shielding average damage = (17*2 + 4)/2 = 19
Stave of Shielding damage over time = 19/28 = .6785

Off Hand
Adamantite Club damage over time = 15/25 = .6

Off Hand
Stave of Shielding damage over time = 17/28 = .607

So level 37 and after Adamantite Club wins for best primary weapon and Stave of Shielding wins for best secondary weapon. I also assume that off hand fires off each time the delay timer ends. So damage would be accumulative .68 + .607 = 1.287.

What is the damage bonus for two handed weapons by level? I know my my Baton of Faith is is 2 at level 28. But my 1 handed weapons damage bonus is 1. Also my formula doesn't take into account random events such as procs and ripostes. Is there something I could add to my formula for ripostes and procs?

Each time the delay timer ends on the offhand the duel wield check is made, if successful then you swing, other wise the timer starts over. The higher your skill the more likely you are to swing with the off hand.

http://wiki.project1999.org/index.ph..._Attack_Chance

If you're trying to create an over arching formula, you also need to take into account misses which will vary greatly depending on your level, skill level, and the mob's level/stats.

I would highly recommend downloading gamparse and just parsing your fights and looking at those "real world" results.

You are correct that Adamantite Club is better in primary and SoS in secondary if those are the two weapons you have available.