Extract N rows from 2 tables (t1, t2) - if t1 doesn't match N rows, then (and only then) extract data from t2
You should be able to do this by using a union to join the two queries together. This assumes that Origin is a value that signifies which table is being queried so that results from t1 take precedence.

SELECT 1 as origin, name, id

FROM t1 WHERE id = 'value' 

union

SELECT 2 as origin, name, id

FROM t2 WHERE id = 'value' 

ORDER BY origin, id 

LIMIT 100