Though, I have not yet made similar project I think it can be done.
Merging two database might be a smaller problem it could be done with a little sweat.
First of all, I would suggest you to upgrade your blog to the same version that of the other blog.
After that, all depends on your scheme to combine the two blog.
Let us assume a complete merge.
After upgrade, dump both databases through phpmysqladmin.
Study carefully the dumps noting structure, syntax and data.
Your main concern will be the posts and comments tables, all the others will need only slight adjustments.
Decide the sequence you want the merge: 1 + 2 or 2 + 1 . Definitely, you should attach the table with less record to the larger one.
INSERT DELAYED INTO wp_posts
(1, 1, ....
The second entry is the post_author. If your are 1 then your friend is 2. Adjust wp_users data accordingly. Now change all.
The first entry is the post ID. If the first database ends e.g. at 465 then start numbering with 466 and adjust all entries. Do not forget to adjust above the insert the AUTO_INCREMENT VALUE= to the next highest value after the combined total.
Unfortunately, you should do this manually unless you have some other method to automate the process. It may be through SQL-query or macro in the editor. I have no knowledge of them :(
Next, your comments:
INSERT DELAYED INTO wp_comments
(1, 1 - 2, 1 - 3 - 1 ...... 1, 2 - 2, 2 - 3, 2 ......... 1, 3 - 2, 3 ......
These mean 3-3 comments in the post 1 and two and two comments in post 3.
That may be a hell of a lot if you have many comments. It may be a day or two hard work.
If you are lucky, some knowledgable people may offer you an sql-query to do the increment from - to automatically. You still need adjust your categories and users tables as well but those are small tasks.
I tried to be thorough but may have missed something. If you people care to revise my post, you can make your corrections.