This is going to be my first tutorial that I made on here. So, let's be gentle as we reach deep inside the game and grab hold of it's assets.
If you watch Abridged, you'll get that.

Awright! This tutorial serves to teach you how to add Dual Skill Support for BDM of characters that don't support Dual Skill.
Thanks to eternity, our x2m characters can now use dual skills!

Tools Needed
dual skill tool
genser
Lazybone's Xml Serializer
Notepad
LibXenoverse [Optional]
You can get the tools here and here

BDM Editing
1. First grab the BAC, BCM and BDM to be edited. For this instance, we'll be using Goku Black Rose. So go ahead and grab these;
[I'll be using HUF files for this tutorial.]
GBR_PLAYER.bac
GBR_PLAYER.bcm
GBR_PLAYER.bdm
HUF_PLAYER.bac or Any other .BAC that supports dual skills.
HUF_PLAYER.bcm or Any other .BCM that supports dual skills.
HUF_PLAYER.bdm or Any other .BDM that supports dual skills.

2. Then follow eternity's tutorial to edit the .bac and .bcm
Eternity's Tutorial

3. Once the .bac and .bcm supports dual skills. We then proceed to Goku Black Rose's BDM.
Drag the GBR_PLAYER.bdm into Lazybone's Xml Serializer. Do the same for the HUF_PLAYER.bdm or Any other .BDM that supports dual skills.

4. Hopefully you used HUF_PLAYER.bac and HUF_PLAYER.bdm. If not then just stick with me for the ride.
Drag the edited GBR_PLAYER.bac into genser. Then open up the GBR_PLAYER.bac.xml with notepad.

5. After that go search for <BacEntry idx="475"
Scroll down until you see the Hitbox section. If you used the HUF_PLAYER.bac as the source. You'll see the codes below.

       <Hitbox>
           <START_TIME value="17" />
           <DURATION value="4" />
           <U_04 value="0x0" />
           <FLAGS value="0x0" />
           <BDM_ENTRY value="32" />
           <HITBOX_FLAGS value="0x2000" />
           <DAMAGE value="10" />
           <DAMAGE_WHEN_BLOCKED value="1" />
           <STAMINA_TAKEN_WHEN_BLOCKED value="9" />
           <U_12 value="0x0" />
           <BDM_FLAGS value="0x1" />
           <U_14 value="0x0, 0x0" />
           <Matrix3x3>
               <L0 value="1.0, -1.0, 1.0" />
               <L1 value="0.0, 0.0, 0.0" />
               <L2 value="0.0, 0.0, 0.0" />
           </Matrix3x3>
       </Hitbox>
If you would look at the <BDM_ENTRY value="32" />. It's value is 32 right? So take that in mind.
If you're using another .bac, the value might be different. So take the value in mind.

6. Now open up the GBR_PLAYER.bdm.xml and HUF_PLAYER.bdm.xml from Step 3 using notepad.
Remember the value in BDM_ENTRY value? Go look it up in the BDM that supports Dual Skill. Likewise, I'm using HUF as a source.
Search for <BDM_Entry ID="32"> in HUF_PLAYER.bdm.xml. Once you find it, copy the entire entry.

7. Paste the copied entry to the GBR_PLAYER.bdm.xml. Paste it just above the </BDM>.

8. Save the GBR_PLAYER.bdm.xml and drag it into Lazybone's Xml Serializer.

And Voila! Goku Black Rose should be able to dual skills now! Without swapping out his .bdm file.
And stuff.. And things...

But upon testing him out. He does his slashy slashy attack when initiating or assisting a dual skill.
Then let's get right to it and fix that up.

Animation Changing
1. Open up GBR_PLAYER.bac.xml using notepad. Look up <BacEntry idx="475"
Likewise, I used the HUF files as the source for the dual skill support. So values may differ.
If you used HUF files as the source. You may see these codes in BacEntry idx="475"

       <Animation>
           <START_TIME value="0" />
           <DURATION value="65" />
           <U_04 value="0x0" />
           <FLAGS value="0x0" />
           <!--0 -> CMN.ean; 0xfffe -> skill ean; 0x5: char ean-->
           <!--Other possible values: 0x9 -> unknown, frequently appears in victim throw entries; 0xa -> fce.ean?; 0xb -> fce.lips.ean-->
           <EAN_TO_USE value="0x5" />
           <EAN_INDEX value="69" />
           <U_0C value="0x5" />
           <U_0E value="0x0" />
           <FRAME_START value="0" />
           <FRAME_END value="0xffff" />
           <FRAME_LOOP_START value="0xffff" />
           <U_16 value="0xffff" />
           <SPEED value="1.0" />
           <TRANSITORY_ANIMATION_CONNECTION_TYPE value="0.0" />
           <TRANSITORY_ANIMATION_COMPRESSION value="0.25" />
       </Animation>
Take a look at the EAN_INDEX value. It's 69 right? That's basically the slashy slashy animation. Also take in mind the EAN_TO_USE value
You can use the LibXenoverse to view the animations in the .ean file. I'm not gonna go into detail with that.
So yada yada you viewed and learned the ID of the animation you want for the initial attack of the dual skill.
So change the value to the one you want.

2. Do the same for the <BacEntry idx="476"
Likewise take in mind the EAN_TO_USE. Make sure it's 0x5. Don't change any other values other than the value of EAN_INDEX.

3. Save the GBR_PLAYER.bac.xml and drag it into genser.

Voila! He now does the animation you wanted!
Thanks to the creator of the tools used in doing this and to you for... You know.. Also gratz if ya got it to work.
Oh yeah... After all the editing.. Apply the files you edited.

I was gonna, add the tutorial for increasing the damage of Dual Skills.. But I guess it's not needed.