Mysqldump multiple tables with where clauses in single transaction
MySQL dump has only a single where clause which makes the use hard in two tables like you do.

One way would be in t1, create a generated column called id that is effectively user_id. 

ALTER TABLE t1
ADD id UNSIGNED INT GENERATED ALWAYS AS (user_id),
ADD INDEX id_key ( id)


Adjust type to match the type of user_id.

ref: https://dev.mysql.com/doc/refman/5.7/en/create-table-secondary-indexes.html