Is an Additional degree required to qualify myself for post doc/industry position outside my area of expertise? Can someone with a Ph.D in field X get a job in academia or industry in field Y, even if the  job posting says "PhD in field Y required?". If not how will going back to get a second post graduate degree be looked upon by academic and industry employers in field Y?  

Background:

I am finishing my PhD in Mathematics and am currently on the job market.   Ideally, I would land a math post-doc position, however, I have also been looking at industry jobs (as some of these would be more satisfactory than say a 4/4 teaching load or 3/3 or whatever it would be), but most of these seem to indicate that I would need a M.S. or PhD in Computer Science.I have a background in math as well as in C.S. (B.S. in C.S., B.A. in Math). 

If I don't get a post doc offer, I am considering attending a school for a master's in C.S. in machine learning or a related field.  Considering I have all the math background, I believe I would be a good candidate for a M.S. or PhD in C.S.  And there are a lot of interesting problems where the two areas overlap.

Is it reasonable to return for a second degree if I don't get my dream post-doc offers, especially to make myself more marketable to industry?  I am seriously considering this option.
Is it reasonable (your original question), yes. Is it necessary to get the job you want, probably not!

"Requirements" for a job are often just loose guidelines, and in some cases are used to scare unqualified, untalented, or under-confident people away from applying. Good employers will usually loosen these requirements (or sometimes even outright ignore all of them) for the right candidate. Someone with a B.S. in computer science and a Ph.D. in math is exactly the type of person who could potentially qualify for a position that asks for a Ph.D. in CS. Requiring a Ph.D. in CS might be used to scare away all the coding ninjas who might otherwise apply. It is all about the story you are able to tell in your resume and interviews. Make the skills you have learned while completing your math Ph.D. appear as transferable as possible. Talk to math grads who are now in industry and peck their brains. Your degree is lucrative if you know how to sell it. I wouldn't recommend getting a second Ph.D just to go out into industry. At the very least apply to a bunch of jobs and see if you get some interviews before you go off and spend 4-6 more years in school.

I know of several people who have Ph.Ds in math and applied math without a B.S. in CS who went into industry. Many of the jobs they took said Ph.D. in CS required. Some companies just want really smart mathy people in these positions.