Site merging vs Organization merging

[Kirk Sutton]

Hello, our business has 2 main teams each with their own Atlassian Org and site. We have a third team that splits their time working for both teams and sort of bridges the gap between teams. There is cross sharing needed between the two sites for confluence and Jira.  

I am wondering what is the best approach for this cross site sharing from a monetary stand point. We don't have any security concerns about the two teams having visibility into everything. 

If we merge organizations and give users access to both sites, are we charged twice as much? If so, it seems like merging sites would be the better way to go? If so, what are the "got yas" to merging sites that we need to account for?

Thanks for any help!