You can have them talk to each other or you can also prep them. In fact, you SHOULD prep them. Many bosses won't understand the importance of a the recommendation letter to this process. Too many of them are used to writing short, superficial work references. This is different.
So, prepping. Something like this:
"Deirdre, I really want you to talk about that bit of data analysis that I did for that last grant report. Do you think you could do that?... Yeah?.... Great.... That's a good story."
"Bill, would you talk about my training skills and the leadership I provide to the volunteer corps? I know you sat in on my last training, and I think that would make a compelling story....You think so too?...Great..."
Yep, that's my work life. Giant love festival.
EDIT: Ooh! That's number 100!