Is an Additional degree required to qualify myself for post doc/industry position outside my area of expertise?
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.