Hi Robert!
Thanks for coming back to us! 🙂
As you might already know, this is not a built in functionality, so you will need to tackle this one by doing some sort of customization.
I did come across this feature idea in our user voice > https://tribe.uservoice.com/forums/195723-feature-ideas/suggestions/5368470-ticket-bundles-multiple-event-passes It may not be exactly what you are looking for, but it surely is an interesting approach to solving your problem so I encourage you to vote for it.
Another thing worth checking out is the Force Sells extension for Woocommerce. I don’t think it will apply to many scenarios, but in your case you could “Sync Force Sells” between the two tickets, so when adding the bonus day it will force the addition of the main ticket to the cart.
I hope that helps you out. If you have other questions let me know.
Best!
Victor