Is it like TWF, but without Double Slice?
So monk level 1, with 18 strength,
(1) flurrying with a two handed monk weapon, or unarmed, or any combo:
I attack at -1 BAB,
I add 4str damage to the first attack,
and 2str damage to the second attack?
(2) And without flurrying, with a two handed monk weapon:
I attack at +0 BAB
I add 6str damage to that single attack?
Is there a feat *like* Double Slice that monks can take?
Double Slice doesn't do much for a Monk, unless your Monk decides to take Two-Weapon Fighting, and use it, but I don't see a strong reason to do that.
Benefit: Add your Strength bonus to damage rolls made with your off-hand weapon.Normal: You normally add only half of your Strength modifier to damage rolls made with a weapon wielded in your off-hand.
When a Monk uses Flurry of Blows there is no "off-hand".
You need an "off-hand" to use Two-Weapon fighting in the first place. Thus if you have no "off-hand" you can't use Two-Weapon fighting.You might not be able to use flurry of blows either, depending on how you interpret "as if using the Two-Weapon Fighting feat".
Except the FAQ states that you can make a flurry of blows with only one weapon.
Monk Flurry of Blows: When I use flurry of blows, can I make all of the attacks with just one weapon, or do I have to use two, as implied by the ability functioning similarly to Two-Weapon Fighting?
You can make all of your attacks with a single monk weapon. Alternatively, you can replace any number of these attacks with an unarmed strike. This FAQ specifically changes a previous ruling made in the blog concerning this issue.
So, you do not need an "off-hand" to use Flurry of blows. It can all be with one weapon.
So monk's flurry of blows ~ TWP + Double Slice?
It is even better than that. Flurry of blows gets full power attack damage on each hit, something that double slice doesn't do (it only handles strength damage).
Also, if you are 2handing, you still get 1.5x power attack damage on each hit.
The advantages of the flurry of blows is subtle, but they are still rather powerful in the right hands.