Decision on connecting Earth Ground to one of Two GND pins
It depends on final equipment ground policy which may differ. In all systems i designed i use GND star connection, which means the following:


All isolated* grounds are connected.
There is only one point of connection, near the power supplies.
Chassis is connected in the same place with a thick wire.


*there are sometimes grounds that somehow depend on other systems and cannot be connected. Most common: minus of diode bridge in AC rectifier. Another possibility- minus of isolated supply for current sensing, which is connected to a motor phase. But since you are not asking about it, we will not discuss it.

Also on each board near each mounting hole there are capacitors that connect the chassis to grounds in AC domain. This is important, because being thick (in relation to PCB copper planes) the case usually has very low inductance and resistance and may be considered perfect ground for noises of all kinds. But on the other hands you don't want any power to pass through the case, so the connection is by caps.

Speaking of ground policy, nice to see that you use optocouplers. Hope you did not forget power filters.