It depends on well they relate to another. If the relation is different in terms of intent or utilization. Even for a related business, unless you are going heavy toward "big brand" with millions of ad dollars toward the parent site.
Split it if you are not going that "brand" route.
A rule of thumb that I would do is, would be "search" your "brand + whatever you are trying to branch into" Without the sub pages being to stand on its own, it will hurt the main site in the mean time like you are afraid of. At least that has been my personal experiences.