Admin center - Multiple small businesses can see each other

Well, you can configure some basic segregation via the so-called GAL segregation/segmentation feature: https://technet.microsoft.com/en-us/library/hh529948(v=exchg.150).aspx

But this will not prevent users from communicating with each other, sharing calendars or Contacts, etc. For that, you might indeed consider configuring separate tenants (although in that case you will loose all the collaboration features).