Oh, thank you for the explanation Corylea! I'm kinda misunderstood first about her statement, my apologies.
As my experience so far, I had a sims that who maxed all of influences, sure it's decay but it's take a quite time to decay, so you can focus into other social group and please a note that you can always maintain the three social group at least you do a interactions with some of members. It's doesn't really matter if you are a rebel and jock at same time, you can always maintain these level even when you are trying to socialize with nerd with the interactions that represent the social group. Example: you can take a photo with nerds that should maintain the jock social level and doing showing gizmo things that also will help to maintain the rebel. It's not always limited into doing activity that can raise the social group influence. I had my sims that his influences decay to 9, but doing a little interactions that represent them should recover them back to 10.
But, for now 'must be reached into level 10 at the same time' is the only what I can tell you, it's like what the LTW say and as well it's trigger the accomplishment. But, I'll sure looking into it again later and I'll let you know about it.
Update: I have tried it, yes it's also count for the LTW when you're at least once reach lv. 10 for any social influences. I have a sims who reached lv. 10 nerds and then she's decay to 9. After that I change her lifetime wish with a reward to ensure this, worked her hard, only focus on her only for reaching the other social groups. And then after several days, when the rest social influences is maxed, her lifetime wishes also accomplished (her nerds influence lv that time is 9, I'm decided not to maxed it to make sure).
