Sure -- the idea is that you want no more than 10 from each table, and then when you have a max of 20 records, re-sort and limit again.
(
SELECT os_comments.comment_author AS author, os_posts.post_title AS title,os_posts.guid AS guid, os_comments.comment_date AS date
FROM os_comments, os_posts
WHERE os_comments.comment_post_ID = os_posts.ID
AND os_posts.post_status = 'publish'
AND os_comments.comment_approved = 1
ORDER BY os_comments.comment_date DESC LIMIT 10
)
UNION ALL
(
SELECT dna_comments.comment_author AS author, dna_posts.post_title AS title,dna_posts.guid AS guid, dna_comments.comment_date AS date
FROM dna_comments, dna_posts
WHERE dna_comments.comment_post_ID = dna_posts.ID
AND dna_posts.post_status = 'publish'
AND dna_comments.comment_approved = 1
ORDER BY dna_comments.comment_date DESC LIMIT 10
)
ORDER BY comment_date DESC LIMIT 10