If I may, let me offer some advice from a person who used to do this kind of thing for a living. The Golden Rule of Clear Writing when it comes to a situation like this, or the use of "who" or "whom," or similar situations -- especially situations in which the proper grammar looks improper, is:
Write around it.
That is, rewrite your sentence to eliminate the possibility of a misunderstanding. In this case, you might write ".....interviewed were Kris Kristofferson, Robert Duvall, and Merle Haggard's two ex-wives" or "...interviewed were Merle Haggard's two ex-wives, as well as Kris Kristofferson and Robert Duvall" or "...interviewed were Merle Haggard's to ex-wives and his friends Kris Kristofferson and Robert Duvall."
Incidentally, that last example, illustrates yet another problem with commas. Some people would put a comma after "friends," but that would be a mistake. A comma in that situation tells the reader that Merle Haggard had only two friends, and their names were Kris Kristofferson and Robert Duvall," while the lack of a comma tells the reader that these are the two of his (presumably many) friends who were interviewed.
One frequently comes across this error in a sentence such as, "I told my daughter, Xanthippe, to have fun at the prom but keep her knees together." This says Xanthippe is the only daughter. If the name is not set off by commas, the reader can reasonably infer that the speaker told his other daughters something else.