All else being equal, solo papers can only be weighted more highly than joint papers in hiring decisions. This is probably especially true at the very top: e.g. a joint Inventiones or Annals paper makes your application look fantastic. A solo publication in either of these journals is a golden ticket for many academic jobs.
Things get much more complicated when one tries to determine how much more to weight solo papers than joint papers. To the best of my knowledge few departments or universities have hard-and-fast rules or even written guidelines about this, so much of this evaluation goes on in the minds of the individual evaluators. The truth is that in some situations jointly authored papers will count to exactly the same degree as solo authored papers, and in other situations the existence of coauthors will cause the work to be substantially discounted.
There is a dramatically increasing prevalence of joint papers in the mathematical profession. Thirty years ago they were quite rare; and they are even more common now than at the beginning of my career, which was not much more than ten years ago. There are now, for instance, certain conferences and workshops in which several people sign up in advance to work on a certain problem under the guidance of a senior mathematician. And then everyone who came to the workshop gets their name put on the paper, even if everything they did was under the guidance of someone else. This is a model much closer to that of the laboratory sciences than what used to be common in mathematics. In my opinion, it is time for the profession as a whole and various groups within the profession to put down in writing some feelings about the merits of joint papers. Of course this will be hard to do since the matter is so complicated: it matters whether your coauthors are "senior" or "junior" to you, it matters whether they have supervised you, it matters what percentage of your papers are joint and whether your papers are always joint with the same coauthors....
Sometimes I see certain publications listed on young people's CV's and think "I find it unlikely that they had a significant intellectual contribution to that work." That's a problem both ways: i.e., people may be wrongly evaluating the merits of this type of work in either direction!