In the Guide, Phil comments on how convenient it is that the killers so often choose to increase their rate of activity just as Mulder and Scully get involved, but this is about the one instance where that could be rationalised away. While Leonard might not know exactly how close to death Lanny is, he must have some idea of how bad his brother's condition is. Consider the museum curator's anecdote about the deaths of the famous conjoined twins Chang and Eng, Eng waking to find his brother dead, and having to lie there, attached to the corpse, for the last hours of his life... Faced with the possibility of a similar fate, doesn't it make sense that Leonard might step up the pace in his search for a new host?