Youtube is the best place to find the right sounding BOV for you. I know that some people specify turbo's with internal wastegates due to liking that particular sound instead of BOV's.
The position of the BOV on piping shouldn't matter, as long as it's not on a different location e.g. between different components. Piping will have the same pressure throughout, though maybe not the same temperature the general working pressure should be the same.
BOV's are rated to different PSI's, you can get higher rated PSI bov's. Rather than having 2, you would be best to try and find one for a high PSI rating I think, as the pressure wouldn't be released equally, assuming your build will require a higher than normal PSI as you specific the possibility of requiring more than one.