Just curious: are you speaking about two [*] JG843A or about two [**] JG844A?
AFAIK if one Switching Fabric Module is installed, then it requires just one Fan Tray (installed on the lower Fan Tray slot), if two Switching Fabric Modules are required then two Fan Trays are required too...and, generally, each Switching Fabric Module should always be installed below (and before) its related Fan Tray...starting from the lower (Slot 10 11, <-- read the Post below about these corrections) dedicated slot (with the upper slot panel filled if the second Switching Fabric Module - Slot 11 10 - and its Fan Tray are not required), the one near the PSUs slots at bottom...and if two Fan Trays are required...they should have the same SKU (so they both provide the same air flow direction).
[*] HPE FlexFabric 7910 Front (Port Side) to Back (Power Side) Airflow Fan Tray (JG843A): red handle, it exhausts warm air out of the 7910 Switch Chassis providing airflow from the port side (Front) to the power side (Rear)
[**] HPE FlexFabric 7910 Back (Power Side) to Front (Port Side) Airflow Fan Tray (JG844A): apparently with a red handle like the JG843A (shouldn't it use a blue handle instead to show that airflow direction is different as happens with the JG684A Fan Tray for the HPE FlexNetwor 7904)?, that SKU was recently added on QuickSpecs (June 2016), the airflow is the opposite with regard to the one provided by the JG843A.
Another curiosity, what display fan, display alarm and display environment and display power commands report?