I don't think anybody builds an airplane you can't keep in balance with an engine and electrical bus failure. I haven't flown the 737 for over 20 years so did a search and found this schematic. The transfer bus has an aft fuel pump for it's fuel tank so when down to one electrical source you still have at least one fuel pump working in each wing tank.
http://www.b737.org.uk/images/schemeelectxt.gif