Well, it's just her face and voice, but as she mentions in the article, she has two stand-ins/acting doubles to read with her for the split-screen takes, or to appear in the scenes where they're shot from behind or out of focus. I noticed a number of shots where they found ways to cheat without using VFX, like keeping the sisters' faces blurry until two of them had moved out of frame. And they didn't really do any complicated interactions between the sisters -- it was mostly just basic split-screen stuff of the sort that movies have been doing since the '30s. So it's nowhere near the level of Orphan Black's twinning effects -- they kept it a lot more simple. The only really impressive part was how they got the actress's timing and delivery right for scenes where the sisters were talking in unison or finishing each other's sentences.