This article explains why the Elo AiO touchcomputers cannot be used as a secondary display

You cannot use an Elo AIO touchcomputer (I-Series, EloPOS, E-Series, etc.) as a secondary display connected to another computer. Elo AIO systems output video but do not receive video. There are no options to input a video source into the AIO to make it a secondary display. 

You can, however, use an Elo AIO to output video to a secondary display as described here and here.