It depends what the CE does. In the event the ones that boost one servents damage should be MLBed as soon as you have enough. The ones that boost team damage or item drops should be kept seperate. After the event is when you should MLB them as the Event Effect goes away. And each event effect only works in the event you got the CE from. But the CE still has the rest of its effects outside the event and those are what really make a CE good long term. Golden Sumo giving +50% starting NP and 10% attack boost is what makes it a highly sought after CE.