Interaction between multiple instances of the same object

How do I...?
2

It’s a known limitation of GD, as GD manages the objects lists for you you can’t do in-list operations, as moving two instances in the same list (same object/object group) to each other’s position.

And yes, as you have deducted you need scene variables to store values temporally.
Working with your EDIT general example, the idea would be:

  1. Pick Seahorse with ID = 2
  2. Save Seahorse position in variables X, Y
  3. Pick Seahorse with ID = 1
  4. Move to position of variables X, Y

Also, take into account that object picking affects the current event and its sub-events only, so you can do things like:

// This event picks the first Seahorse
Conditions: Variable "ID" of Seahorse is = 1
Actions: No actions

        // This sub-event picks every instance and then the second Seahorse, changes the pick for this event and its sub-events only
        Conditions: Take into account every Seahorse
                    Variable "ID" of Seahorse is = 2
        Actions: Do = Seahorse.X() to the variable "X"
                 Do = Seahorse.Y() to the variable "Y"

                // Sub-sub-event, still referring to the second Seahorse
                Conditions: No conditions
                Actions: Do + 90*TimeDelta() to the angle of Seahorse

        // Sub-event, still referring to the first Seahorse
        Conditions: No conditions
        Actions: Move Seahorse to Variable(X) ; Variable(Y)