You would have to export the database of one blog and manually add its entries and comments to the database of the other blog. And, depending on the amount of entries, that may extremely difficult.
While manually inserting the posts and comments into the database of the chosen blog, you will need to ensure that the
comment_ID fields continue to increase by increments of one in a steady pattern with no duplication. You will also need to make sure that each
comment_ID corresponds with the proper
post_ID of the post that it belongs to, especially if you had to change that specific post's
post_ID when moving it to the new database.
In my opinion, it would be better to start using only one blog now, and just close all comments, pingbacks, and trackbacks on the other blos and refer back to it if need be.