Sure. That's what is know as a star configuration. You can stack the cables from both instruments onto the connector of the controller. Most GPIB cables have a male and female connector for this reason. As long as you don't exceed the total length of cabling in the 488.2 spec. The other type of wiring is called a daisy chain. With this, you would have a cable going from the controller to instrument a and then another cable going from instrument a to instrument b.
