There is no priority if it is a series system.  All the oil is going through both sections of the system.  All we are deciding is what path it travels.  This will determine how much back pressure is on the part which it goes through first.  

If the fans run at 750 psi and the apron runs at 1500 psi, the system will run at 2250 psi either way, the difference is whether the fan motor has 1500 psi of outlet pressure and the apron none, or the apron has 750 psi of outlet pressure and the fan none.    Most motors can take 750 psi outlet pressure without any big problems, even without a case drain.  If you put 1500 psi on the outlet of a motor, you better have a case drain, and it better be a pretty high end motor, or you are going to have leakage problems.    Furthermore, in the event you transport the rig before starting to unload, and the chain is stalled, when you engage the apron, the apron will see full bypass psi....if it is first, it is the only thing that sees it....the fan is freewheeling at that point anyway...if the apron is second, then when you attempt to start it, you are going to spike the pressure in the fan system to bypass pressure...this is a sure way to screw up motor seals in your fan motors.