I read that Lucky Egg gives you double XP for 30 mins. I just want to know if it's possible to activate 2 Lucky Eggs at the same time. If yes, what is its effect? Do you gain x4 XP or waste the second one?
No, Lucky eggs (I guess you're talking about these) won't stack. Actually you can't even use a second Lucky egg after using the first, since you'll get the "You've already applied this type of enhancement." message (thanks @jdero for the info)