It's easier in a way to give a theoretical answer, which is that you squeeze the income stream for the two people out of what the employed partner has set aside for retirement. Rather than have a survivor benefit delivered through a cross-subsidy to all plan members, you in effect say that the income stream—normally that of the male spouse—has to be sufficient to cover both lives. You then try to eliminate the cross-subsidy that way.
One of the problems in the divorce situation is that even if you split the assets out by one formula or other at the time of divorce, the labour market experience of the divorced wife after the divorce may not be comparable to that of the divorced man. So you end up with an unequal situation. I would say, too, that to some extent—and while I appreciate your thinking that as we look to the future we want mechanisms that involve a limited degree of cross-subsidization—you've thought about it primarily with respect to taxpayers. I would remind you, though, that these things also happen inside private arrangements. There are always limits to them.
For example, you're in a workplace pension plan that has a survivor benefit built into it. Single participants in the plan subsidize married participants, generally speaking. People with short life expectancy subsidize people with long life expectancy. Granted, there are tolerances—beyond which you theoretically and practically don't want to go—but I would just say that within certain bounds, people seem willing to put up with this.
I think the short answer to your question is that you have to get two streams of income out of one accumulation.