We have been practicing Scrum for some years now. Due to reorganisation our team was rebuild. The new team consists of application managers, developers and content managers. We can not completely dissolve the knowledge islands, since it is e.g. unlikely that the content managers would start to learn programming and vice versa.
We try to do planning poker and sometimes multiple people can estimate. But often it is quite senseless since only one or maximum two team members can really estimate the work to be done.