What is the difference between a single and double union pressure reducing valve/regulator?
A double-union device, and it doesn't matter if it's a pressure reducer or valve or whatever, should always be used where it's not at an end of the line.  That way if it ever needs to be removed for replacement or service, that can be done.
The single-union will not have enough clearance to be removed since it's likely the pipes on either side of it will not be movable.  As you unscrew the non-union side, the union side will be pushed into the pipe and likely prevent you from getting it off.
Interestingly, I've seen a water company service guy reach down into a valve/meter box, unscrew the two unions on the valve (it had broken) and screw a new one in without having to shut off the main.  It didn't take him 30 seconds to do it.  They didn't want to shut off the main because my street only had a single shutoff for the entire length of the main vs. hookup by hookup shutoffs which are standard these days.  He could never have done that with a single-union valve.