Wall 2 Wall Comments (similar to Facebook)

I'm having some problems with a query to retrieve data similar to how a wall 2 wall comments area would would (eg, shows the 2 users discussion). Anyway I was able to do it one way but thats with data from just 1 table. On my site comments on users profiles are split into separate tables (an early decision, not sure why).

For testing I tried it for 2 users whos profile comments are both in the same table using this query:

 SELECT u.user_id, u.username, c.comment_id, c.user_id, c.friend_id, c.time, c.message
FROM users u, user_comments_a c
WHERE (
c.user_id =1
AND u.user_id = c.friend_id
AND c.friend_id =2
)
OR (
c.user_id =2
AND u.user_id = c.friend_id
AND c.friend_id =1
)
ORDER BY time DESC
LIMIT 20 

Code (markup):

Whilst that works fine in that situation with both users sharing the same comments table, how could it be done if the comments are stored in 2 different tables?

Such as

• user_comments_a (fields: comment_id, user_id, friend_id, time, message)
• user_comments_b (fields: comment_id, user_id, friend_id, time, message)

 

[Extra]

Sorry, forgot to add that this is for MySQL.