You may have misread the literature as to the insulation type or you were reading sch. 80 pvc. instead of sch.40. The type of wire you want to use for your project is Thhn/Thwn stranded wire. It's the wire you see on the small spools and the power wire rack at the big box stores. It comes in several colors for hot conductors, neutrals (always white) and green for equipment ground. 1/2" sch.40 rigid pvc (RNMC) will accept 8 #12 thhn/thwn conductors. Heres a table for your reference.
Using a shared neutral (multiwired circuit) is commonly done but you must understand the concept in order to keep from having some unsafe issues with neutral loading, possible high voltage on equipment if the neutral opens and annoying issues when using shared neutrals with gfci's. Also gfci for multiwires is expensive in that you either must purchase a 240 volt gfci or install gfci at every outlet using pigtails off the common neutral wire.
For these reasons I would not do a multiwire I would recommend running 2 20 amp circuits, this would be a run of 5 wires. Two hots, two neutrals and a shared equipment ground. Also understand that two 20 amp circuits provide 4800 watts of power, which is considerable if your just wanting a few lights and receptacles and maybe a fountain pump....that sort of thing. One 20 amp should be plenty but only you know the power requirements of what you want to operate.
These circuits outlets must be gfci protected... if you protect them before they leave the house and go underground with gfci breakers or a gfci receptacle located just inside then the conduit only needs to be buried 12" deep otherwise you must bury it 18" deep and install a gfci receptacle/receptacles where you want by location to be code compliant.
Normally your wiring will hit a receptacle box first where you will install a gfci and then wire your other receptacles and switches with a wiring run from the gfci load terminals. Allowing you to install regular grounding type 3 prong receptacles everywhere else and protected by the single gfci.
Do not use 90 degree elbows except above ground. When you are transitioning from vertical going into or out of the trench to horizontal use 90 conduit sweeps and when changing directions in the trench beyond what the flex of the conduit will allow.. use sweeps.. they come in 45 and 90 and a few others. A sweep is just a gradual transition bend and you will see these in the conduit fitting aisle at the big box.
Be careful of box size when you install gfci's they need a big volume box so get the deep weather proof ones. You also need to install "in use" covers like these.....this is code for outside now days.
If you are new to pvc conduit remember to not run the wires in the conduit till the glue dries as it is damaging to the wires when wet. We normally complete the conduit installation and install a pull string as you go if doing it diy. Or a fish tape works if the run isn't to long with too many bends. A friends help is great. When you take wire off the spool don't let it spiral off roll it out on the ground or put it on a wire rack that you can make yourself so that it comes off straight and not coiled.
When you buy your receptacles get the "backwired" type these will make terminating the stranded wire a breeze. You simply strip the wire 1/2" to 9/16" and insert straight in to the backwire hole then tighten the screw till the pressure plate clamps the wire. Otherwise get spade terminals and crimp them to the wires if you are terminating under screws. Do not confuse backwire with backstab receptacles.
To explain sch.80 vs. sch. 40 PVC conduit......80 is used in areas that local codes require for physical protection and is normally very short lengths where your conduit is exposed leaving and going into the ground. In your case you don't need to adjust to a larger size sch. 80 before entering your sch.40 down in the trench. It is necessary sometimes to increase the size of sch. 80 because it accepts less wires than sch. 40 does due to a thicker wall....thus why it is required for physical protection areas instead of sch. 40. Now having said all that sch.40 is allowed by a lot of inspectors in residential applications for the entire conduit run. so only way you will know is to ask the codes department in your area.
this is a good place to familiarize yourself with the different types of pvc fittings and boxes and sweeps etc.... there are 5 pages ..a E986-car is called an LB and is used when entering a dwelling with a wiring run, sweeps are on the last page. I dont like the flexible ones hard to get your wires to pull through them because of the ridges.
Hope this was some help, hard to cover all the bases