Help with collaboration across site collections

I have departmental teamsites in seperate site collections like so:

/teams/IT
/teams/HR

Before I get too far into this way of setting  up our sharepoint collaboration sites I want to be sure that its going to work.   I setup our dept teamsites this way because i wanted segregation from a security point of view.  

However there will be instances where a manager from IT and a manager from HR need to collaborate on a document type of project.  That document may need to remain inside an HR document library.  Or do I create a seperate site collection for managers of all depts?  

So the big question is by putting our teamsites in seperate site collections have we made it difficult to collaborate across departments?  I need some help thinking this through.  Im hoping someone could give me some scenarios on how we can maintain cross departmental collaboration while keeping departments in seperate site collections.

Originally I had one site collection for the organization and all dept teamsites as subsites but scrapped it due to not wanting to affect other teams search and also if I needed to do a db store via site collection.  Plus I sleep better at night knowing someone doesnt gain access to an HR document there not supposed to be seeing because of a mistake in permissions.

Please help me think this through.