select user_add from tab1 where time_add BETWEEN $time_start and $time_end
union all /* без ALL выведет только уники */
select user_add from tab2 where time_add BETWEEN $time_start and $time_end select user_add from tab1 where time_add BETWEEN $time_start and $time_end
union all /* без ALL выведет только уники */
select user_add from tab2 where time_add BETWEEN $time_start and $time_end