Varbūt šādi:
SELECT 1 AS KasTasIr, * FROM chati WHERE chati.owner=$uid
UNION
SELECT 2 AS KasTasIr, * FROM forumi WHERE forumi.owner=$uid
Vai arī iespējams, ka MySQL nepatīk *, gadījumos, kad esi radījis mākslīgu kolonnu:
SELECT 1 AS KasTasIr, lauks1, lauks2,...,lauksN FROM chati WHERE chati.owner=$uid
UNION
SELECT 2 AS KasTasIr, lauks1, lauks2,...,lauksN FROM forumi WHERE forumi.owner=$uid