If you have a machine in your operation that performs two tasks in sequence on each Work Item (e.g. Setup and Weld on a welding machine), it may be best to use the Multi-task Timing feature. Using this feature, you can control an Activity's process time by defining multiple tasks (e.g. Setup and Weld) that occur at that Activity. Each task can have its own process time, resource and image. You can find more information about Multi-task Timing here.
Alternatively, the machine may be simulated as two separate Activities directly connected together with no buffer between them. However, without giving Simul8 an instruction, these two Activities could both contain a Work Item at the same time (the same machine being Setup and Welding at the same time).
Simple solution: Create the two Activities and join the Setup Activity to the Weld one with a routing arrow. Then select both Activities (drag a box around them) and click the right mouse button. Select “Create Group”. Now go to the Group's dialog box (click the Groups button on the Data & Rules tab) and enter 1 in the “Limit Work Items in Group” box.
(Remember to also click on each Activity in turn and enter the appropriate timing information for how long it takes to Setup and Weld).