Eh, I disagree. Synch is able to copy powers without harming or weakening anyone while Rogue is more... vampiric in her absorption. I never liked Carey's interpretation that Rogue could "borrow" a power without a real cost, even if she didn't intend to harm the person she was absorbing they should at least not be functioning at 100%.
Anyway this is a moot point, since there is overlap between Jean and Lorna, Scott and Shiro, etc... if you want to get technical. Just because there is some overlap doesn't mean the powers act in the same way.
I actually really like that they are one the same time, it allows Duggan to show their specializations and differences in a clear way.