A splitter cable will not allow 3 devices (2 computers and a switch port) to communicate at all, much less with ISE. Switched Ethernet just doesn't work that way.
When multiple devices are connected to a switch port (either with a downstream unauthorized hub or a phone with a PC connected, etc.) the switch configured with ISE as the RADIUS server for dot1x authorization control will detect the multiple MAC addresses and behave according to the configured policy (authenticate each separately, authenticate only one data and one voice domain device, block all bu the first connected address, etc.)