Together means at the same time
Select collaboration when the stage requires the assigned people simultaneously. The engine intersects their productive availability, excluding breaks, leave and commitments. One person's free afternoon cannot replace another person's missing time.
Why a small interruption can have a larger consequence
Suppose a one-hour review needs Nick and Keith. Their shared Monday window ends at noon. A four-hour urgent request pushes the preparation into the afternoon, so the review now needs Tuesday's shared window. A dependent project then waits for Tuesday's completion.
The request consumed four productive hours. The project finish may move much further in calendar time because a shared window was lost. Those effects compound; they are not a precise additive breakdown of delay.
Choose independent work when the resources do not need to work simultaneously. Read Why this date to check the resulting gate. See interruptions and movement.