Yes you can. In such case you would have to teach two TCPs and switch between them during program run. And in most cases you would need four different payload/CoG settings: both grippers empty, both grippers loaded, gripper 1 loaded (gripper 2 empty), and gripper 2 loaded (gripper 1 empty). You can find an example of a dual-gripper setup in our free machine tending e-learning here: academy.universal-robots.com/free-e-learning/e-series-e-learning/e-series-application-track/
No. The payload wizard estimates the center of gravity of both the gripper and workpiece as if they were one physical object. The payload wizard cannot estimate the center of gravity for only the workpiece.
Yes, you can design and build your own end-effector. We don't have a specific template, but you can join our developer network where you can find all the information you need and get help from other developers: www.universal-robots.com/developer/