is there a way to setup a relation between two parts in an assembly that have a slider mechanism so that the move in opposite directions as you drag one or the other?
Probably the easiest way is to define a gear connection. Just use the same diameter for both of the gears and they will move at the same speed.
not dealing with gears, this is a slide, so not sure how one would go about defining a gear connection, plus I do not see a gear connection as an option as a component placement. are you talking about a gear mechanism?
Here's a quick example:
You will need two of these with the diameter set the same:
Demo files are attached (Creo 3.0)