My experience, along with my fiancee, having lived and worked in 3 different major US cities in the past 5 years, is that assumption just isn't reasonable. It's hard enough to find two good jobs in a household in the same _CITY_ right now, much less the 10-15 mile radius of biking. In areas where it might be possible, the cost of housing is so high that it eats up that savings.
1. New York: Lots of jobs in tight proximity and good public transport, but even in the "cheaper" boroughs, you're still probably paying $250K or more extra for a small family residence.
2. Los Angeles: Cheap housing. Owning less than two cars with two careers? Possible, but very, very difficult. Short commutes? Unlikely.
3. San Francisco: Expensive housing. Expensive car ownership. Jobs spread throughout the peninsula.
Not to mention what happens when you have kids to transport.
What we really need is to lift restrictions on new housing construction in dense urban areas like New York and San Francisco. The only way you can have both high job density and moderate housing prices close together is to allow enough vertical construction to reduce competition for housing to reasonable levels. Until you do that, you're inevitable going to see a large part of the population, especially younger / latecomer families, forced to live away from the job centers so they can reduce housing costs.
I think this is a pipe dream, unfortunately. How could you sell such an idea to the existing homeowner who comprise the voting base?
In my view, the forces behind not allowing more vertical building are really developers who want more sprawl, because they want to buy up unused land at the fringes of a city and build houses on it rather than have to assemble blocks from existing neighborhoods.
The unfortunate answer in all this is that the best way to reduce commuting is to make it harder to commute. Don't build more roads to subsidize long commutes, make work parking more expensive, and favor public transport in allocation of lanes and space. Europe is already doing this, and some cities in the US (like San Francisco) are also doing it to some degree.
I suspect, however, that we won't really get serious about it here in the US until gas hits about $15 a gallon.