They probably drop spit on the inner raised platforms, and then egress to the outer back platforms for scream.
That means they have to tank high though, because the spit takes longer to despawn than the duration of a single tanking cycle.
Also, since 2.0, anomalies can spawn on the outer-ring platforms at the far left and right extremes, which means that any tanking there has the potential to be disrupted by anomaly killing. My group used to tank on the raised platforms between the tentacles, backing up to drop spit high and then going back down for the scream. We had to change this with 2.0 though, dropping spit on the unused tentacle platform, since we had a large number of instances where an anomaly would spawn on the platform with spit on it.