People that have more than one line usually buy one line from each provider so they can talk "for free" between the same providers. It is not uncommon for people to ask you what is your provider so they will mark it and call from a phone that uses that provider when trying to reach you.
We also have a high number of people with phones that allow multiple providers with one device.
Business and personal cellphones are not uncommon either.
[1] http://www.ibge.gov.br/home/estatistica/populacao/censo2010/... [2] http://extra.globo.com/noticias/celular-e-tecnologia/quinto-...