At this moment it is not possible to configure selected monitors for a Horizon session on IGEL OS (in this scenario you have two different Horizon Sessions running on one thin client). It either starts fullscreen on all the monitors or on a single monitor.
You can only do this manually by adjusting the following Horizon Client settings on the IGEL desktop:
Display -> Full Screen - Single Monitor (that is the default) to Full Screen - All Monitors and select the screen you want (see attachment). Can this please be put into the profile settings for Horizon?