The Berakha (blessing) that one recites before the Bediqath Hamess (Hametz - search for the leaven) can cover his search in multiple houses. The physical going from one house to another is not considered to be a Hefseq (interruption). Despite different opinions, the custom is that even if they are far away from each other, the same blessing covers the other houses or stores or places of business.
If the head of the household wishes, he may have other members of the household stand next to him when he recites the Berakha and they then disperse to different parts of the house to check their own area. They do not recite a separate Berakha but rely on the one made by the householder. The same rule applies if he hires someone not from his household to assist with the Bediqah (checking).
The head of the household must also check at least a portion of the house since his obligation is greater than that of his Shaliyah (agent). There is also an opinion that he is not permitted to recite the blessing if he does not check at all. As such, he should first check a portion of the house after reciting the blessing and then let his Shaliyah continue.
(See Sh. 'A. 432: 2. Kaf Hahaim ibid, 22-24, 26, Mishnah Berurah ibid 7,8)
