So the game does most of the math for us by displaying overall DPS, but I was trying to come up with a way to quickly make a decision between DW and 1hxbow with a quiver. Please dont take this as absolute truth, as my work has not been checked. Also, I am ignoring all the useful mods on the OH/quiver because though they rarely are the same taking them into account for usefulness is a little outside the scope of what I was comparing.

Lets define these varibles:

D1 = DPS of MH 1hxbow

D2 = DPS of OH 1hxbow

A1 = Attacks/Sec of MH 1hxbow

A2 = Attacks/Sec of OH 1hxbow

AQ = IAS % of quiver (which I'll write as 1.AQ below)

Your DPS when DW should be the weighted average according to attacks/sec of both weapons

DW DPS = (D1 * 1.15 * A1) * A1/(A1+A2) + (D2 * 1.15 * A2) * A2/(A1+A2)

Your DPS when using the MH with a quiver

1H DPS = D1 * 1.AQ * A1

So now I am going to make several assumptions to make all this easier. I am going to assume that both MH and OH dont have +Attacks/sec mods or +IAS mods, which will make A1 and A2 both equal 1.6. Also, I'm going to assume a +12% quiver, which at level 59 is the fastest one I've found with other good mods. Then I am going to set both equations equal to one another to see at what point they reach equilibrium. This gives us:

1.792 * D1 = (.92 * D1) + (.92 * D2)

Subtract from .92*D1 both sides and divide the rest by .92 to isolate D2 (rounding) and you get:

.95 * D1 = D2

This means that in order for DW to be a good idea your OH's DPS needs to be at least 95% of your MH. If your MH has a higher attacks/sec then it can be a little lower, but if your OH has a higher attacks/sec it needs to be even closer. A faster quiver (I've seen 15% on the Auction house) means they need to be closer. If I screwed up somewhere please let me know.

Lets define these varibles:

D1 = DPS of MH 1hxbow

D2 = DPS of OH 1hxbow

A1 = Attacks/Sec of MH 1hxbow

A2 = Attacks/Sec of OH 1hxbow

AQ = IAS % of quiver (which I'll write as 1.AQ below)

Your DPS when DW should be the weighted average according to attacks/sec of both weapons

DW DPS = (D1 * 1.15 * A1) * A1/(A1+A2) + (D2 * 1.15 * A2) * A2/(A1+A2)

Your DPS when using the MH with a quiver

1H DPS = D1 * 1.AQ * A1

So now I am going to make several assumptions to make all this easier. I am going to assume that both MH and OH dont have +Attacks/sec mods or +IAS mods, which will make A1 and A2 both equal 1.6. Also, I'm going to assume a +12% quiver, which at level 59 is the fastest one I've found with other good mods. Then I am going to set both equations equal to one another to see at what point they reach equilibrium. This gives us:

1.792 * D1 = (.92 * D1) + (.92 * D2)

Subtract from .92*D1 both sides and divide the rest by .92 to isolate D2 (rounding) and you get:

.95 * D1 = D2

This means that in order for DW to be a good idea your OH's DPS needs to be at least 95% of your MH. If your MH has a higher attacks/sec then it can be a little lower, but if your OH has a higher attacks/sec it needs to be even closer. A faster quiver (I've seen 15% on the Auction house) means they need to be closer. If I screwed up somewhere please let me know.