You can’t combine them in one command but you can execute both commands at the same time and output to the same file.
mysqldump -u user -p --no-data db > structure.sql; mysqldump -u user -p db table1 table2 >> structure.sql
to avoid having to enter the password twice you can do -ppassword (note the lack of space!). Also use --no-data in the first command or you end up with the data as well. -d isn’t needed when you’re doing just one database.