How do "scientific couples" (from different fields) find positions?
Many universities (like the one where my wife and I work) have dual-career hiring programs explicitly to address this issue.  It's definitely worth asking—carefully—whether the universities you are considering have such a program.

Here's how our system works.  Suppose Department X offers a faculty position to Partner A, and later department Y offers a faculty position to Partner B.  Then Partner B's salary is paid 1/3 by department Y, 1/3 by department X, and 1/3 from a general campus fund.*  (Note that X and Y may be the same department.)

So Department Y has a significant financial incentive to hire Partner B.  On the other hand, Department X must be willing to pay extra for Partner A; in practice, however, once an faculty offer to A is actually on the table, most department chairs find it hard to refuse to help hire B.

This is why you have to be careful how and when you ask.  It's illegal in the US to discriminate against a job candidate because they're married—we're not even allowed to ask—but it is completely legal to hire a cheaper candidate over a more expensive one.  If department X already knows that you have a two-body problem, they also know that hiring you will be more expensive.  The safest strategy is not to mention that your partner needs a position (or even that your partner is another academic) until an offer is on the table.



*When I originally posted this answer in 2012, this financial arrangement was de-facto permanent, but my university has recently refined its policies.  As of June 2018, the joint funding arrangement is permanent (“recurring”) if Partner B has a tenure-track faculty position, and limited to three years otherwise.