A.C.
Two suggestions ... one is to let her take either her morning nap or afternoon nap while you run errands, as others have suggested.
The other option is to start working on moving her nap time closer to his, and vice versa, to get them napping at the same time. Start laying her down in the morning closer and closer to noon and start laying him down closer and closer to 11am ish. till they meet in the middle :) or just start working her nap up to noon like his. Keep her up for about 15 minutes more every few days till you get them closer together for nap time. You can also start laying him down 15 minutes earlier till they are both going down at the same time. I hope that made sense LOL
Good luck :)