I would use "are" there. I suspect you would hear "is" from some.
Doing business in Russia and doing business in the US are/is completely different.
Can I use both here?
I would too- there are two things and we're separating them by saying that they're different, so using the plural verb makes more sense to me. I agree that some speakers may well use is, though.
There are hundreds of millions of native speakers in many countries, so it's often hard to have absolute agreement on something. Sometimes it is a matter of choice whether we think of two things as one unit and singular or as two individual things. Here, I think there are two things (doing business in the US + doing business in Russia), but both Mike and I can imagine someone thinking of it as all one thing (doing business). I don't think there's a very strong case for using is, but I can see some people using it.
I don't think a native would think of the two things as one and choose "is" because of that. I think it more likely that someone talking spontaneously would lose sight of the subject by the time they got to the end and just match "is" with the closest noun.