Entertainment Weekly explains:
Filming a scene with both Vincent and Frankie in the frame is
difficult enough. Those scenes would typically start with Franco as
Vincent. The hair and makeup on the twins are subtly different but
distinctive enough to necessitate a break for the switch. Production
would run through the scene with Franco doing Vincent opposite a
double playing Frankie.
Franco, a fan of improvising, would have his brain working overtime
during those scenes, thinking ahead to his Frankie time. “Normally,
when I’m working with Seth Rogen and we improvise, you just
improvise,” Franco said. “You throw out the line, but I wasn’t playing
Frankie at that moment. In order to make the space in the performance
so that I could respond as Vincent, I would have to then tell Will
[his double] to try this line. It was weird.”
Once the production felt satisfied with the Vincent side, Franco would
head to the makeup chair to become Frankie, and his double would
change clothes. It’s usually during this downtime when the crew would
capture plates — or empty frames, free of actors, that replicate the
camera movements for visual effects to build off of in post. Once
everyone was out of makeup, they’d redo the scene incorporating the
improv they liked from the Vincent takes.
From there, it’s just a matter of some VFX wizardry in
post-production, combining the two Franco takes the showrunners like
best. The effect, historically achieved through a split screen, might
be one of the oldest in visual storytelling, but it still has the
ability to feel like magic.
He also directed two of the show's episodes!
“They thought it was impossible, not only to direct yourself as an
actor, but to direct yourself as twins,” Franco said over the phone
weeks after our visit. “It’s insane. But when you saw me, I was at the
height of my multitasking. I sort of felt like this was things going
the normal course, like, ‘Of course, I’ll direct myself as twins.’ Now
that I look back, I do think it was pretty crazy.”