Combining Explain and Union
As @jkavalik said in a comment, according to the MySQL docs UNION can only be used to combine the outputs of SELECTs, which EXPLAIN is not.
However, you can line up the raw outputs of each EXPLAIN in a text file using the MySQL commands in a shell. Run this once for each EXPLAIN statement:
mysql -u $USERNAME -p$PASSWORD -Bse &quot;[EXPLAIN STATEMENT GOES HERE]&quot; &gt;&gt; explains.csv
From the MySQL docs:

-B: Print results using tab as the column separator
-s: Silent mode. Produce less output.
-e statement: Execute the statement and quit.

This will give you a tab delimited row and &gt;&gt; will append to the file specified at the end, creating it if it is not already there.