Over the past four elections, the Libertarians have averaged a bit more than three tenths of one percent of the Presidential vote, while the Constitution/Taxpayer Party has averaged around one tenth of one percent. So even if they pooled their resources, on a good year they would be lucky to get one half of one percent, or probably less than a Nader type would pull from the Democrats. It's a long, hard road.
On the other hand, the Libertarians and Constitution Party will poll independently lower if Ron Paul is the GOP nominee, because most of them won't be voting independent of the GOP except on local races.