Daniel Larison and Mark Thompson make the case that libertarians -- and, presumably, paleoconservatives -- would be better off if both Ron Paul and Gary Johnson ran for president in 2012. Larison further argues that "it would largely help to have more competitors in the Republican primary defending civil liberties, arguing against unnecessary wars, and presenting an uncompromising challenge to Republican enabling of government profligacy and debt."
Purely from the perspective of the message sent in debates, which was clearly a big part of Paul's influence in 2008, that could be true (if they are both disciplined about focusing on these arguments rather than letting their competitors or the moderators bait them into ideological sideshows). But there is also an electoral impact to consider. If you look at the national polls, Paul is polling roughly where Pat Buchanan was before the 1996 primaries. Buchanan didn't win the nomination but he did have a breakthrough year. Paul, whose supporters are more organized to build on any successes in the Republican primaries, may be similarly poised.
If Gary Johnson ran and Paul didn't, he would have to build his support from scratch. And while many of us hoped he would be able to take a broadly Paulian platform a step further based on his record as governor and more mainstream image, very little in what I've seen of his public appearances or interviews in the last year bears this out. Johnson would be much easier to reduce to a libertarian cartoon character and, for that reason, I think he'd probably be a step back.
If Paul and Johnson both run, it will spilt the already limited Republican vote for their ideas at a time when Paul's support might be reaching critical mass. While it may seem to be logical to just look at their combined vote totals when assessing GOP support for anti-statism or non-interventionism, a top three finish by Paul in Iowa or New Hampshire would have more impact than looking at a significant Paul-Johnson combined vote. There's a Buchanan tie-in here too: Alan Keyes detracted from Buchanan's presidential bid and probably cost him Iowa in 1996.
Now, Keyes didn't keep Buchanan from finishing second behind Bob Dole in the race for the Republican nomination. Presumably, the Paul and Johnson campaigns wouldn't be as antagonistic toward each other as the Buchanan brigades and the Keyesters were. But all other things being equal, it still seems like something worth avoiding.