I have a single daughter, then a pair of boy-girl twins!
I feel that it can go both ways, and most likely your 2nd twin is going to be a middle child with first born tendencies.
It would be expected that your first children followed rules and instructions very well, because as first time parents we wanted to get things right and were tighter in control. however, the need to carve a niche of themselves in the family still exists, so even though they are twins and people expect them to develop similar personalities, they are in fact going to be quite distinctly different from one another!
Your twins are definitely going to be an attraction (mine are!) whenever you're out, and I think they would make a good elder sibling team and look out for their younger brother!
The bottom line with twins is to ALWAYS treat them as unique individuals, never as a pair, while the other would be to make sure they don't get too much limelight or their single sibling would be the one blended into the background!