They and its predecessors have been practically doing it since the 1990s, by using uniform branding and sharing some programs.
The RockDétente branding had been in use since 1990, when CITE-FM in Montreal adopted that branding. It would soon be extended to Telemedia's other AC-formatted stations (Astral acquired Telemedia in 2002.)
Similarly, CKMF-FM in Montreal first adopted the "Énergie" name in the 1980s, and would soon followed to its similar stations in the Radiomutuel group (which Astral acquired in 2000). Énergie became the similar-sounding "NRJ" in 2009, after Astral licensed the name and imaging from the NRJ Group, an European broadcaster.
As for EZ Rock -- it was a branding begun by Telemedia in the 1990s, then carried over to Standard Broadcasting, then to Astral, through a series of station acquisitions. While it's still used by Astral's stations in Western Canada and Ontario (plus a few Ontario stations owned by Rogers), EZ Rock's former flagship, CJEZ in Toronto, flipped to classic hits earlier this year as CHBM, "Boom FM", using the branding and format used by Astral's French-language rimshot stations in Quebec (though adapting it for English).