The singers began dating their partners, who are both members of British dance act Diversity, in 2012, and they both became single again around the same time earlier this month, reported Daily Mirror.
Thirlwall split from Sam Craske and Nelson called it a day with Jordan Banjo.
"It happened very recently for both couples... While it's strange timing and quite a coincidence that both girls were dating boys in the same dance troupe, it seems to have stemmed from the amount of time they are able to spend together, because of their conflicting schedules," a source said.