Sounds to me like going to sleep at 6pm and then waking again til 10pm is that her own body clock is telling her that at 6pm it's still just a 'nap', and 10pm is 'bedtime'. Her rhythm requires one last nap in the day for some reason....
Perhaps as others have suggested trying to keep her awake, perhaps even for just 30mins longer, that gap in between the two sleeps will lessen and then eventually disappear?
Gradually eek it out a bit longer til you put her down for the night?
Another thought- Is there are a lot of disruption around your DD2 once she's down for the night? DS2 was getting into a similar sort of pattern.
What works for us though the last 3 months is that I have both mine in the same room, but put DS2 (6mths) down to sleep at 6.45pm after both their baths, and take DS1(3yrs) into our room to have stories in our bed.
We did it so that DS2 would stop getting disrupted. Falling asleep when put down, then there was so much going on nearby that he would wake, and then could not be settled again for hours. Now I do not need to worry about grumbles coming from their bedroom, if there are any, and vice versa.
With this, DS1 sees it as his special thing to get stories in our bed and he has now had the message drilled into him that the bedroom is the sleep room (all toys are elsewhere, so that helps). Both quiet in same room by about 7.15ish.
And I think that has made a MASSIVE difference, as the last three nights DS2 has slept 7 til 5/6/7 !! I know this will not stay for long, but I can start to see light at the end of the tunnel.... until teething, or a cold, or something else that ruins it all(!)