It depends on what current you run them at. 2 Is a hard number to work with without using individual drivers, which generally means lower driver efficiency and more wiring complexity.
Something like HLG-120H-C500 is the smallest 94% efficient driver I know of that would allow you to run 8 of them in series at 500mA...
HLG-120H-C700 is is 94% efficient at 700ma and supports 6 in series.
You could get HLG-80H-C700 and run 3 in series at 700mA 91.5% efficient driver
You could get HLG-60H-C700 and run 2 in series at 700mA.. 90.5% efficient driver. There's your 2, but it'd be at 700mA, and slightly less efficient driver.