Mysql – How to combine “SELECT” and “REPLACE INTO” when combining two MySQL tables into another

I've got data with measurements split between two different tables, but the data doesn't need to be present in both:

mysql> SELECT * FROM t1;          mysql> SELECT * FROM t2; 
+----------------+-------+        +----------------+-------+
| measurement_id | value |        | measurement_id | value |
+----------------+-------+        +----------------+-------+
| 1              | data1 |        | 1              | NULL  |
| 2              | data1 |        | 2              | data2 |
| 3              | NULL  |        | 4              | data2 |
| 5              | data1 |        | 8              | data2 |
| 8              | data1 |        | 9              | data2 |
| 9              | NULL  |        +----------------+-------+
+----------------+-------+

My users need to be able to combine the two tables into a third table, depending on their choice of whether data1 should take precedence over data2, whether NULL data should be copied, etc.

An example of one such possible user-requirement combination is "fill t3 with data from t1 unless there is non-NULL data from t2 to replace NULL or non-existing data from t1." The resulting table would be

mysql> SELECT * FROM t3;
+----------------+-------+
| measurement_id | value |
+----------------+-------+
| 1              | data1 |
| 2              | data1 |
| 3              | NULL  |
| 4              | data2 |
| 5              | data1 |
| 8              | data1 |
| 9              | data2 |
+----------------+-------+

The way I implement that condition is the following SQL commands:

INSERT INTO t3 SELECT * FROM t1;
REPLACE INTO t3 SELECT * FROM t2 WHERE t2.measurement_id NOT IN (SELECT measurement_id FROM t1) OR t2.measurement_id IN (SELECT measurement_id FROM t1 WHERE value IS NULL);

There are other example, more simple or more complicated, I can give, but they all rely on first copying all data from one table into another, then doing a REPLACE INTO to overwrite data on conditions.

My question is: How can I combine the above INSERT and REPLACE INTO commands into a single command so I'm not copying data from t1 that will just be replaced in the second command?