Boothill & Firefly team?

Idk if this has been asked but its been bothering me for a while. I have both Boothill and Firefly, complete with their best teammates so far (E1S1 Ruan Mei, Harmony MC, and E6 Gallagher).

This MOC cycle (n previous) has fire weakness on one half and physical on the other half. How am I supposed to use both characters when they share the same exact teammate? Is it even possible to use them both?

I somehow managed to clear last MOC with the help of the break bonus but yeah its not working again this time.

Note: I do not own Acheron or Kafka so I cant make use of the other weakness (Lightning) that the boss has. My Boothill is E0S1, Firefly E0. No Sparkle, no Robin, no Topaz, no Himeko.