Ooh, I know this one.
One way you could do it is choose one of them to be the actual boss. It doesn't matter which, and it'll have no effect on anything, I'll name it "Benben". Wherever you create the boss object "Benben", make another enemy but make this an ordinary enemy (use OBJ_ENEMY, not OBJ_ENEMY_BOSS), which I'll call "Recliner".
Change all your render tasks to have a variable parameter, such as "target", and everywhere the task would normally have "Benben", replace it with "target". Call every task requiring "target" twice, once with "Benben" and once with "Recliner".
If you want them to have a single health bar (like the Prismrivers) there are a couple ways to do it. I think the easiest way is double the amount of damage your shot does, then create only a hitbox for "Benben" at "Benben"'s location. Immediately after creating that hitbox, create another hitbox for "Benben" but create it at "Recliner"'s position. In your end task (the task that deletes the boss when it's health runs out) delete both "Benben" and "Recliner".
If you want to make each boss have a separate health bars (like in the Tsukumo's opening non spell) take your end task and give it the variable parameter "target". Call the end task twice, once with "Benben" and once with "Recliner". In the end task, remove the "CloseScript" function. Make another task that will only close the script once both enemy's health is equal to zero. Create a hitbox for each enemy and place it at their location. When you create "Recliner", also set it's health and damage rate.
If you want to make each boss have their own health bar, but have the spell end when either of them dies (like in the Tsukumo's spell cards) go about it as if they each had their own health bar (the paragraph above) but don't remove the "CloseScript" function from your end task.
Sorry if this is confusing, I hope I helped.