Assign two different pins, one to each of the CPW "grounds". If they are both set to be a ground reference for the center conductor pin, then this will equalize the potentials, and you don't need to physically connect them.
This is, of course, only at the port. If you need to suppress the coupled slotline mode further into your circuit, creating air bridges (or vias to a conductor backing) are really your only options. However, if your antenna feedline are short, you won't need to worry about this.