Why in the world would you want to do this? Think about it, you have similar data in two different columns that should be in one. If a value in (c) is the same as (b) which enables it to have a FK link to (a) then why not merge the columns and save yourself some headaches?
This is a normal task. Think about a table with some attributes, and you want to assign 2 of those attributes to one item. The esiest (only?) way to do, is to take two columns, and put a foreign-key on them.
In my case, i have a table with all the users in my system, and i want to save the user that created the file, and the user that last modified the item. So i have two columns with the different userids, and i'd like to but a foreign-key with "set null on delete" on them.
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