I think to prepare for shadow lords mode they should do a double XP event so everyone who already has it gets 4x XP try to get people up to 50 before shadow lords mode.
Um if you have double xp plus a double xp weekend,that is 3x the Xp.
This is true, each multiplier adds x1 XP
Base XP gain = 1x
Ultra XP bonus + 1x = 2x
XP event + 1x = 3x
Paid booster + 1x = 4x
So to get quad XP you need to have S3 ultra on a 2x XP weekend AND buy an XP booster from the store.