Okay, that's just plain silly. If we were talking about different countries that would necessitate different TLD's (.com and .co.uk) then there's a case to be made for international SEO. If it's a company working within a region in a specific state then it makes absolutely zero sense whatsoever. The devil's advocate argument (that is completely misguided) is that two sites equals two opportunities to rank for regional keywords. The only way this argument works is if they were going for "Anaheim pest control" and "Huntington Beach pest control" and owned EMD's for both (AnaheimPestControl.com and HuntingtonBeachPestControl.com). Considering that regional keyword phrases have very small keyword opportunities I would be reticent to even suggest that approach.
This view of SEO also blocks them off from regionally based generic keyword phrases. The term "pest control" is going to get a lot more searches at a national level than "Anaheim pest control" or any regional derivation. They would be better suited to have one single domain that consolidates authority over time than to "split" that authority over two sites. All of this completely ignores the statement about two different designs, which strays into brand identity, but is another no-no if you talk to any marketer worth their salt.
Finally, I hope that when you sell a 5-page site that the end user has some sort of CMS that they can add more to it with. If you're selling static HTML pages with zero ability to add new content selling with SEO in mind then you should quit your job immediately because your company doesn't know how SEO works. Microsites for print, radio, TV, banner, or SEM campaigns do much better when they're small and constrained, but by their very design they do very poorly from an SEO perspective. I regularly recommend two-page microsites to paid search clients because you can get a much better conversion rate (5-10% on average versus <2.5% for a large site); however, I would never tell them that tiny little sites like that will have any real chance at ranking in the SE's.