I'm a twin (with non-ID sister) with 10yr old twins (b/g twins) too so am watching carefully.
For me, I don't remember it being intimidating although of course it must be, and actually it was good that she was there - are your girls going to be in the same class? We were, and when we were streamed into ability groups, we were in the same group too. We still had different friends though. One issue was that her group of friends was slightly more trendy than mine, we used to bicker about that.
I think they'll both get organised quite quickly, even if they're not in the same class I'm guessing their timetables won't be massively different. You might need a chalkboard or something to keep tabs on what they each need.
I suppose competitiveness became more pronounced because secondary school was more results driven and I do remember feeling second best if I didn't get a higher result than her, but it was generally swings and roundabouts. I did better in some tests, more in others. I wasn't on the netball team (and she was) which caused a slight inferiority complex for a while - I suppose you just have to keep an eye on them.
The only other thing that grated was that even though we are not ID, we were / are very difficult to tell apart so teachers (who had less time to get to know us) would sometimes refer to us as "the X twins". Hopefully that isn't an issue if yours are different but its worth perhaps building in 1:1 time if you can so they're not always together.
Looking forward to reading what everyone else has to say!