It can be done, but there is no provision for external regulation in these devices, so the power will not be equally shared between the two devices. It is probably a better idea to use the older ADuM6x0x (ADuM620x/ADuM640x) or ADuM5x0x (ADuM520x/ADuM540x) devices; they are capable of 3x the power and are about equivalent to 2 of the new devices in terms of PCB footprint.