To do this dual mystic arte, you need to have a battle action called “Friendly Link”. You can get this battle action by finding all the monoliths, if you need to know the location of the monoliths you can check the link below this video main page or link below.
The other requirement is your human character that initiate the combo (Sorey/Rose) need to have at least 9 BG. You can reduce the BG requirement if you have a certain skill which can reduce the BG usage.
Make sure all of your Seraphs have enough BG to use their mystic arte (3 BG minimum).
To use the friendly link skills, you need to hold certain direction on the D-pad button and then keep pressing O button while doing a mystic arte.
Up => Lailah
Down => Edna
Right => Zaveid
Left => Mikleo
Now to execute the dual mystic arte, you need to call your Seraph following the orders which written above (Lailah, Edna, Zaveid, and Mikleo). While Mikleo is doing his mystic arte, keep pressing O button. If your main character still have at least 6 BG, then he/she will use the dual mystic arte: Elemental. See the video below if you’re still confused and need step by step to execute this perfectly.
Watch the video below if you need tips to raise your BG.