Word for two things that are opposite yet the same

There is a "figure of speech" that may fit:

I'm damned if I do and damned if I don't.

There is Catch-22 also:

(idiomatic) A difficult situation from which there is no escape because it involves mutually conflicting or dependent conditions.

Probably the most common idiomatic usage for OP's context is

two sides of the same coin

The Cambridge Dictionaries Online definition is...

If two things are two sides of the same coin, they are very closely related although they seem different

But personally I prefer this one from Yahoo Answers...

It means that the same person or subject or idea can be viewed two different ways.
[emphasis mine]

In OP's specific example, several hundred writers have in fact made the point that Fascism and Communism are two sides of the same coin. They mean both are oppressive/totalitarian regimes. Much the same idea is eloquently put by George Orwell in the closing words of Animal Farm...

“The creatures outside looked from pig to man, and from man to pig, and from pig to man again; but already it was impossible to say which was which.”

One phrase that expresses this:

a rock and a hard place.

Or to make a general statement:

All roads lead to ...

For instance, no matter how governments begin all roads lead to debt and dissipation.