Continuing Mark Polino's weekly This weeks is a vote on adding support for multiple companies within one database. This is the first of mark's suggestions to vote on that I'm slightly sceptical of.
Generally, a second company is dealt with in a separate database when it is a separate legal entity. If it's not then it's handled within the account framework (i.e. the first segment is defined as the company code); I can't really envisage the technical approach that might be needed for a third way. Perhaps it's just a failure of the imagination and someone can enlighten me.
If you'd be interested in seeing this change made, please go to Connect and vote.