I had a small discussion earlier today about this topic, and was told these abilities were possibly mutually exclusive because the devs couldn't get clones to cast Lacerate. That's fine and all, I'm not here to yell at anyone to simply make it work.
The real reason I'm suggesting this is because Mirror Image doesn't make clones repeat any spells, it just gives you clones. So, why should they be mutually exclusive right?
Though, if clones stay unable to cast Lacerate, Echo will have to become mutually exclusive instead. However, I think this opens up for at least a bit more flexibility in Assassin's tree, by letting Acrobat grab clones for extra tankiness if it wants.