Is the Double Skull Tap only used for Boss RC Loop Stations?

No. You can use the Double Skull Tap to manage two separate pedals with tap tempo input. You can even use it in combination with a Boss RC Loop Station. For example, with the Double Skull Tap, you can control a Boss RC-3 Loop Station on one side and a Boss DD-7 on the other. These would have separate tap controls. If you want to control two devices with one switch, then you’ll want to look at the Skull Taps with Dual Jacks.