select top 10 * from (
SELECT   PostsId AS id, PostsTitle AS Title,PostsDate AS SendDate, 1 AS Type
FROM      BbsPosts
WHERE   (PostsUserId = 'admin' AND IsDeleted=False ) 
UNION ALL
SELECT   ReplyId AS id, ReplyTitle AS Title,ReplyDate as SendDate, 2 AS type
FROM      BbsReply
WHERE   (ReplyUserId = 'admin' AND IsDeleted=False )
)
order by SendDate desc

------解决方案--------------------
如果是按 SendDate 排序
select top 10 * from (
SELECT PostsId AS id, PostsTitle AS Title,PostsDate AS SendDate, 1 AS Type
FROM BbsPosts
WHERE (PostsUserId = 'admin' AND IsDeleted=False )  
UNION ALL
SELECT ReplyId AS id, ReplyTitle AS Title,ReplyDate as SendDate, 2 AS type
FROM BbsReply
WHERE (ReplyUserId = 'admin' AND IsDeleted=False ) )
order by SendDate desc