I sometimes am configuring a tool when I realize I need to go back and re-run another tool. It is helpful to pull that previous tool from my GP history so I can just make the tweak and get on with it, but unfortunately the only way to do so is to knock out the tool you have going. Opening something from GP History replaces the active GP tool in the GP pane, losing any parameters you've set. A R click --> Open as additional tool (or similarly worded) from the history pane would be a fantastic addition for this workflow.