It's pretty convoluted from what Ive read.Check out this guy's article, it explains it in a pretty straightforward way.
There are some restrictions that that dudes post doesnt cover:
. Player has to be rank 130 or higher
. Secondary monster has to be max level and rarity 5* or higher
. Secondary monster has to come from the REM or a Collab REM, no pal machine stuff
. Secondary monster has to have all awakenings unlocked
There are some restrictions that that dudes post doesnt cover:
. Player has to be rank 130 or higher
. Secondary monster has to be max level and rarity 5* or higher
. Secondary monster has to come from the REM or a Collab REM, no pal machine stuff
. Secondary monster has to have all awakenings unlocked