Answer

Apparently, one should recite a blessing for each structure, since this is a blessing over a commandment, and each structure involves a separate commandment. But if one performs the same commandment several times in succession, one recites the blessing only once (as with the blessing over Torah study, according to the views that treat it as a blessing over a commandment, or sitting in the sukkah). Still, this can be analyzed from several angles. The common assumption is that one does not recite a blessing over several mezuzot in the same house because there is no interruption there. Therefore Beit Hillel, Yoreh De'ah sec. 289, para. 1, writes that walking from house to house does count as an interruption, and therefore one must recite another blessing. But I am doubtful about this. There is also room to discuss whether the blessing in the same house exempts all the mezuzot regardless of interruption, simply because it is the same house. According to that, with two houses one should recite two blessings. But that too seems questionable. Bottom line: I would not recite another blessing if it is all being done at one time by the same person.