But if two people are speaking Belarussian or Ukrainian to each other, how much can you, as a Russian, understand?
It depends. I had very little experiance with Ukrainian and no experiance with Belorussian. When people speak slowly, I can understand them, but I don't understand films. That's the question of practice. If you hear Ukrainian for the first time, it's hard. If you watch TV for two weeks, you start understand nearly everything. It is important to know the most common different words and to know some phonetical differences. Like Russian "o" - Ukrainian "i".
I have also been wondering whether it isn't a problem for some peole in Belarus and Ukraine that some signs are only in Belarussian / Ukrainian.
Of course, it's not.
I asked LOTS of people in Belarus if they could speak Belarussian and I met only two people who said they could speak it.
When I was in Ukraine I did not ask about it.
The situation in Ukraine and in Belarus is very different.
In Ukraine there are probably more people whose mothertoungue is Ukrainian than those whose mothertoungue is Russian. But I'm not sure.