For me, the term Singer doesn't mean just Singer. He has a team of people he's comfortable with, Chris Lee, Ottman, Guy Dyas, etc. They've already done a lot of the research in SR, so it's a matter of the villain and his sets and costume. I'm not even going to pretend to know United Artist deal, but I guess Singer will be involved in editing and scoring. In the digital age, it's a lot easier to be multi-task.
Of course, this all under the assumption SR2 happens.

t:
Of course he's not doing this alone. He has his team just lik any director. But the guy needs to be there to over see things. He needs to involved in the editing process , he needs to be involved in the scoring. He needs to give the VFX people approval of shots etc.
If we are to believe that everything is going as planned and singer will do Valkyrie and Mayor and then Supes , here's how the will probably go
1 Shooting has already begun on Valkyrie. Singer shoots the movie while at the same time his team of Mayor is beginning pre-prod.
Because he is in Germany the Mayor-team will either be with him in Germany or they'll set up a high-speed internet connection and SInger can talk with them .
2 Singer finishes shooting on Valkyrie and begins post-prod.
At the same time he gets starts to get ready to begin filming on Castro Street. By this time the superman script will most likely be approved by WB so they'll begin prep on that as well.
Question :
Explain to me how he can A) be busy with post. on Valkyrie , spec. during the editing and scoring sessions , B) also begin filming Mayor , C) begin prep. on superman sequel.
3 If everything goes as planned ,singer will probably finish work on Valkyrie two months before release. Usually with these big projects work continues right till the last moment meaning that Singer would finish up on Valkyrie between June-July. of '08.
At the same time he'll be begin post on Mayor while still finishing up on prep. with the sequel.
And again he will needs to be there when prepping the sequel . He needs to give approval to the costume departement , discuss early VFX tests , do the animatics etc.
Question :
Explain to me again how Singer is going to post. on two separate movies while at the same time doing doing and/or finishing prep. on the sequel.
Don't take this as an attack towards singer. I just don't see this happening realisitcally speaking. Two movies can be handled by a director but not 3 movies.
Maybe , just maybe Singer might to handle. post. prod. on Mayor IF he begins shooting the superman sequel.
But again from a realistic POV it just wouldn't work to handle two project at the same time .
I'll use Spielberg again as an example. With Jur. Park he wasn't even involved with the post. prod.. Lucas was doing it. And when Spielberg made WOTW and Munich in '05 he first did WOTW and only when the movie came out did he begin prod. on Munich.
If singer wants to make Mayor , i doubt he will be making it before prep on the superman sequel.
If he can do it , great for him. I'd like to see that but so far i just don't see it happening.