Morning bus ride is $850/year.
Afternoon bus ride is also $850.
Buy a morning seat at $850 and add an afternoon seat for $350.
If I use the Related Products method, someone (who doesn't actually want a morning seat) can go to the morning product, and click on the afternoon add-on product and purchase an afternoon seat for $350, without having to buy a morning seat.
If I use the product option method, information about the afternoon product is not displayed because it isn't really a product, it's just an add-on.
Furthermore, I assume that the inventory counts for optional add-ons and actual products are not linked, so controlling capacity would be a manual operation.
My question is: How to make the purchase of a product dependant on the prior purchase of another product?
For instance, a discount for the afternoon product if you order a morning product.