Its random what island will appear. Though in my game I seem to get 2-3 at a time, usally if you do get more than one, its commonly a basic Mirage Forest/Island/Cave/Mountian and a Mirage with some of the other gen legends.
UPDATE: (It seems the more people you get passes from VIA the PSS you more Mirages that will appear)
You can increase the Spots that appear VIA the PSS passing system.
The Spot with the lowest chance to appear is Crescent Island, this is the Mirage spot with Cresselia.
Here is a list of the Mirage Spots with Other game legends and have the following Pokemon.
Trackless Path: Raikou, Entei, Suicune
Pathless Plain: Cobalion, Terrakion, Virizion
Nameless Cavern: Uxie, Azelf, Mespirt
Fabled Cave: Reshiram (Omega Ruby Only), Zekrom (Alpha Sapphire Only)
Gnarled Den: Kyurem Update: (You need both Reshiram And Zekrom for this one to appear)
Storm Clouds: Thudurus (Omega Ruby Only), Tornadus (Alpha Sapphire Only), Landorus
Dimensional Rift: Palkia (Omega Ruby Only), Dialga (Alpha Sapphire Only), Giratina - UPdate (For Giratina to appear you need both Palkia and Dialga and for the Rift to appear you need the Physic Trio of Uxie, Azelf, Mespirt)
Sources: Own Experiences and Official Hoenn Region Strategy Guide.