In another question it was established that while the two originate from two different planets (Coruscant and Stewjon respectively), This can be explained away by the fact that, at least in EU canon, humans from Coruscant colonized everywhere with sleeper ships. My question is why, if this naming convention is so wide-spread, then why do the majority of humans have single word names? Is this a cultural thing or just random baby naming?
We don’t know
To the best of my knowledge, no one has mentioned any connection between Obi-Wan’s heritage and Qui-Gon’s. Part of this is because, even in Legends, there’s not much information on Obi-Wan’s childhood on Stewjon.
It’s perhaps worth noting that the source establishing that Qui-Gon was born on Coruscant is new Disney canon (Ultimate Star Wars) and thus in Legends it is possible that Qui-Gon also was born on Stewjon.
We don’t have any particular reason to think that Obi-Wan and Qui-Gon come from a common culture background merely because they have hyphenated first names, though. Consider, for example, Jar-Jar Binks (or for that matter Roo-Roo Page), who has a name that follows the same general design as “Obi-Wan Kenobi” and “Qui-Gon Jinn,” despite being a Gungan from Naboo. Admittedly, these names rely on reduplication, making them a slightly different pattern, but it does raise the point that hyphenated names might simply be a common feature in the galaxy.