Definition of single-ended vs. double-ended SMPS
The term "double-ended" is used for only Flyback and Forward converters that using two series-connected active switches (BJT or MOSFET) instead of one. Each switch is placed on each end of primary winding of the power transformer and they are all connected in series. Traditional ones that contains only one switch are called "single-ended". 

Actually,  the term "double-ended" is not widely used, some academic articles used this term. These converters are commonly known as "two switch flyback converter" and "two switch forward converter", respectively.

PS: I'm writing this from my smartphone. I wish I could add a schematic for each, but you can Google "two switch flyback" or "two switch forward" to get a lot of useful info.