First off, don't go down the 'everyone I know has babies who sleep through' route - that way madness lies! Plus, I bet you anything it isn't true.
Second, twins are different. As you know, when one of them wakes, it disturbs the other, and you get caught in a vicious cycle.
Third, your son sounds 'settled' in his sleep pattern, so I should leave it as it is.
Lastly, your daughter. That's your real problem here. At six months, I would be trying her on just two naps a day (not sure my twins ever had three naps, but then it is all a bit of a blur). Though if you do try this, put your son on the same schedule. Having them on different schedules is going to make your life hell (you know this)!
My son has always needed less sleep than my daughter. I think the way we handled it has meant that - sadly - we went by his schedule and she spent her first year a little bit sleep deprived (that was not the intention). BUT it also meant that life/sleep started to make a bit more sense at six months.
Whatever happens, good luck. A lot of people here - me included - know how hard it is with twins. And yes, I can reassure you that they will settle down soon.
And you might consider moving this thread to Multiple Births. Lots of good advice over there!
LLx