ZBrushCentral

opacity slider

is it possible to make a slider that will change the Front Opacity and Back Opacity together?

In theory, yes, but you’d need to determine how you wanted each slider to change.

I would just like them to be linked to the same value, I’ve no clue how to do this though.

There’s lots of info on ZScripting here:
http://www.pixologic.com/docs/index.php/ZScripting