How can I schedule a post on multiple subreddits at once?

You can do this quickly in the Post Scheduler.

Enter all subreddits you want to post on in the Pick subreddit(s) to post on field.

Next, create the content for your posts.

Finally, enter a title and pick a post time. You can click Add post if you want to use a different title or post time for specific subreddits. Then select at least one subreddit in each group (it turns orange).