Use one union as you said, but this is usually how we install the blowout if there is both one on the inlet and outlet side:
When you want to blow out the sprinklers and the mainline, use the blowout hookup on the outlet side of the PVB. Then when you want to blowout from the PVB and back to the line in the house, use the hookup on the inlet side. When you blowout, the air should never go through the PVB, as usually the valves connecting it to the PVB itself have the blowout hookup attached to it. This means if you want to blowout the mainline from the inlet hookup to the beggining of the mainline in the basement, that you have a ballvalve teed on the mainline so that one end is free with nothing attached to the end, this way when you blowout the mainline in the basment, that the water has someplace to escape.
THE BLOWOUT HOOK-UP
This was for a customer that didn't want any backflow for some reason, so I came up that they're going to somehow need a blowout hookup. So, I took an air hook-up from a tire ( I forget what they are called), drilled a hole through a cap, just big enough for the tip to fit through, and insert it through the hole until the black stop and sealer stopped it. then take the nut and thread it on the opposite side of the the sealer and stopper (the sealer and stopper will both be on the underside of the cap) and tighted it until it is hand tight with a crescent wrench. The sealer will stand the pressure at all times. then, take a small piece of pipe, slip it on to a slip x slip x slip tee, then slip the cap with the air hookup to the small piece of pipe on the tee. Then, slip it on the mainline pipe in the basement right after the shut-off valve. This may sound kind of complicated, so if you want a picture I will try to illustrate it on paint on my computer if you want to see it. e-mail me at akooima17@hotmail.com if you want to see the picture I drew for it. But, I don't reccomend using this if you have a PVB or any other backflow device, as it should be used only when you don't use a backflow device.
This website is also really good with the blowouts and the whole installation of a sprinkler system. If you need more info on blowouts and winterization, try this site: www.jessstryker.com
Hope this helps and if you need more info, e-mail me at
akooima17@hotmail.com
As for the blowout fitting, use the 3/4 inch size.