The National Electrical Code does not list a minimum cover requirement for communications circuits, so in theory you could put the conduit at grade level. Irrigation control and landscape lighting has a minimum cover requirement of 6" (150 mm), so I'd probably go with that as a basis.
Since you're not specific about the electric power circuit, I'll discuss a few different situations.
120 volts, GFCI protected, 20 amperes of less
If this is a residential application (which I'm assuming it is, or you wouldn't ask the question), and the circuit is 120 volts or less, and the circuit is GFCI protected, and the maximum overcurrent protection is 20 amperes. The minimum cover requirement is only 12" (300 mm). In this situation, you could bury the electric power at 12", and the communication circuit at 6". This allows 6" of separation, which is likely enough to avoid interference.
Ideally you'll probably want to bury the power circuit at 18", and the communications circuit at 6". Then you'll have 12" of separation, which should be enough to prevent interference.
Over 120 volts, or not GFCI protected, or greater than 20 amperes
If the power circuit is 240 volts, or not GFCI protected, or the maximum overcurrent protection is over 20 amperes. The minimum cover requirement is 24" (600 mm). In this case, you can bury the electric power at 24", and the communication circuit at 6". Then there will be 18" of separation, which is surely enough to avoid interference.
You will want to limit the voltage drop in your DC distribution wiring to 5% or less of the 12V. That would be equivalent to a 0.6V drop. An online wire size calculator suggests that for 600 feet at 3A that you would need to use 1 AWG copper wire.
Cost of that size wire suggests that you may want to consider distributing the AC mains voltage instead and converting to 12VDC at the load points. The reason being is that the amount of voltage drop in the wiring follows the amount of current through the wire. 30-40W at a line level voltage results in less than 0.5A down the wire and 5% voltage drop at 120VACrms makes room for a larger allowable voltage drop than the 0.6V at 12V. The net result is that a much more reasonable wire size can be utilized.
If you use optical fiber cables they could be in the same conduit as DC cables. You may want to separate them from AC mains cabling just because it seems like a good idea to keep AC mains by itself.
Best Answer
Wrong conduit for the job
First off, you are looking at the wrong kind of conduit for this task. NEC 348.12 point 6 prohibits the use of FMC underground:
Given your desired trench depth, you want to use rigid metal conduit (RMC) instead, as even liquidtight flexible metal conduit (LFMC), which can be buried, requires 18" of topcover.
You don't need wires that fat, either
Second, your 8AWG wires are quite a bit too large; for a general-use receptacle at a mere 85' distance, 12AWG will suffice. (Rule of thumb: if the one-way length of the circuit is longer than the nominal voltage, then you start need to worrying about voltage drop.)
Other tip: put the GFCI at the house
Another tip here is to put the GFCI inside, at the house, where it is better sheltered from the weather. While a good weatherproof box and while-in-use cover will do their darnest to keep rain out, they're still not as good as the roof of your house at it. Also, don't forget to fit the bottom of your receptacle box with a weep hole (1/8" is fine) to both let water out and keep air pressure inside even.