This is what we do if I have to bath both of mine by myself, if it helps..
6.00ish we all go into DS's room to get undressed. I undress DS down to his nappy, then run the bath while I help DD undress. I also get DD to help by carrying towels through to the bathroom etc.
Both are in the bath by 6.10 and we do teeth in the bath. I get them both out by 6.20ish. Back into DS's room. I dry off DS and get his nappy on, then dry DD. While DD gets her PJs, chooses her book for bedtime and her toys for her bed I dress DS. This obviously involves DD doing a lot by herself though. She's in and out of the room though so basically I'm giving her little jobs to do which all help for bedtime. When DS is ready I put him in his cot with a few toys to play while I get his bottle and help DD do her last bits. Would your DD play in the cot for 10 minutes at this time? If not, could you dress them both together (pj top for DS, then it's DD's turn for her vest, then DS's pj bottoms etc?) and then just spend 5 minutes doing the last bits for DS while DD plays? Or just bring her with you?
By 6.45 I'm doing DS's bottle anyway. DD chills in her room with books while I do this but if she gets bored I let her sit on the floor of DS's room and listen while I read DS a story. She's only allowed to stay if she stays quiet! She understands that DS needs quiet before bed. For the last couple of minutes she has to go into her room and wait on her bed with toys or books while I sing to DS and actually put him to bed as 6.55ish. Then I go into her, read her stories and turn off the light. She's normally in bed by 7.10.
The only difficult bit is if DS won't settle. If I can possibly leave him (he's doing a mantra cry) I do until I can get DD in bed. Otherwise I guess I'd try and settle DS and sort of go between the two of them until I'd got DD in bed. I just have to be firm with DD and tell her i can't hang around sometimes cos DS is upset but promise to come and check on her once DS is settled.
Hope that helps?