Two Nings combine into one JR site?
Installation and Configuration
An interesting question

Just had a quick look at the Import code to remind myself how it all works and there's a 13 character Ning ID associated with all Ning members and that is used to tag all other items to their owners, the same as Jamroom does with profile and user IDs. Assuming that ID is unique for every Ning member, across all Ning sites, then theoretically yes, we could merge archives into one site as the Import module uses the same Ning ID to associate imported items with imported profiles.
However, I would want to add some additional checks to the import process as JR expects all profile names, user names and email addresses to be unique as well, and I don't think that is guaranteed withing two or more Ning archives, so would want to check for that and take appropriate action if found.
hth