From parallel two-faced lines, the end dancers and the center dancers are going around the square together, in the same direction, so they can hold hands the whole way. But the centers must still drop hands with each other. From parallel two-faced lines, All 8 Circulate is the same as Couples Circulate.
All 8 Circulate can also be done in columns. All eight dancers move one spot along the single path that goes through all of them. From columns, All 8 Circulate is the same as Column Circulate or Single File Circulate.
Note that in all of these cases, the dancers are circulating along the natural path or paths for the given formation. So some callers may say simply Circulate. The additional words ("All 8") can help make clear that all eight dancers should be doing a circulate, in situations when the previous action might lead dancers to expect that only some of them should be active. They also serve to emphasize that the caller wants this default kind of circulate, when some other kind of circulate (e.g., Split Circulate) would also be possible from the same formation.