Johnson and Paul are going to split the politically conservative (what gets called Libertarian these days) vote unfortunately.
Paul has his following, and I doubt the hard-core will go to Johnson... but I bet Johnson picks up more new folks. It's really too bad. They should get together and agree to one bowing out.
I personally think Johnson is a much more viable candidate in a general election: he's younger, fit, relatively articulate, and doesn't do the goofy old-man stuff Paul sometimes slips into.