Mysql selecting from two tables

You want UNION.

(SELECT tr.full_name, tr.headlines, tr.content, tr.stamp, tr.person_key
 FROM tbl_transactions tr
          JOIN persons p ON p.person_key = tr.person_key
          JOIN teams t ON (pp.membership_id = t.id and pp.membership_type = 'teams')
 WHERE t.team_key = '')
UNION
(SELECT tr.full_name, tr.headlines, tr.content, tr.stamp, tr.person_key
 FROM tbl_transactions_bk_2012 tr
          JOIN persons p ON p.person_key = tr.person_key
          JOIN teams t ON (pp.membership_id = t.id and pp.membership_type = 'teams')
 WHERE t.team_key = ''

Just put the join condition in the WHERE clause:

SELECT t1.*, t2.*
FROM table1 t1, table2 t2
WHERE t1.id = t2.t1_id

That is an inner join, though.

UPDATE

Upon looking at your queries: In this particular case, there is no relation between tbl_transactions and tbl_transactions_bk_2012 (i.e. joining these on person_key is meaningless because there is no relationship between the two tables in the way that (say) tbl_transactions and persons are related).

Then, you should use the UNION approach. Trying to join the first query to the second using either JOIN or FROM xx, yy WHERE xx.id=yy.id is meaningless and won't give you the results you need.

By the way, in the future, put your current query/attempt in your post - as you can see it will prevent you from getting answers that aren't appropriate for your question (as my first attempt was).