There should be only one stock fuel filter and it is in the frame rail under drivers seat area.
IN would be correct for the fuel line coming from the fuel tank, fuel is flowing IN to the filter and OUT to the engine.
No, on a 1991 there is a check valve inside the fuel pump, it isn't a separate piece.
And there is a Fuel Pressure Regulator(FPR) on the fuel rail/engine that is connected to the fuel Return line that runs back to the fuel tank, so there are 2 fuel lines.
The FPR and fuel pump's check valve are what hold fuel pressure in the system when engine and fuel pump are off.
Could that second fuel filter be on the return line?
Not sure why it would be there.
Could you be mistaking the FPR(round cylinder) with return fuel line attached, as a filter?
FPR also has a vacuum hose attached.
FPR is a spring loaded valve and should hold fuel pressure up to 40psi then it will start to open and send extra fuel and pressure back to the fuel tank.
You can carefully pinch off Return fuel line, i.e. vise grips but don't crush it, and then cycle key on and off 2 or 3 times(fuel pump runs for 2 seconds each time key is turned on).
Then see if fuel pressure holds.
If it does then replace FPR, it is leaking
If fuel pressure still drops then Check Valve is the problem, so have to replace whole fuel pump.
There are slight differences in designs but this is a picture of an FPR: http://www.partsgeek.com/assets/dimage/full/1200403.jpg
1 vacuum hose connection
1 connection to Fuel rail
1 connection for return fuel line