How to configure discount in ecart object?

I am having trouble figuring out how to structure a discount in ecart. I'm not sure how to go about it.

I have a situation where two different classes are given on one day. One class in the morning and one in the afternoon. Each class is usually taken and paid for separately. What I need to do is have a discount if both classes are taken together on the same date. There are only two classes held on any given day and both are priced the same amount. How do I offer a discount of X$ if someone registers for both classes?

I have looked at the guides and on the forum, but none of the information seems applicable. I thought maybe if I had a column in the database where both courses on a certain day had the same discount code I could match the discount number somehow, but I don't really know what I'm doing and the whole thing is making my eyes glaze over. Could you please point me in the right direction to implement this? Thanks!

In the Discount settings, one of the triggers is to check for a certain by ID, You could use that trigger to check for the 2 items by their ID to trigger the discount.

Thank you! I added a discount code column to the db. But since I'm not clear on how to do what I'm trying to do, I'm still trying to get things to work.
Questions:
When I add the Discount column to ecart what is the correct out put type? Number? Text? Integer? Currency?

I've tried a bunch of different combinations, but I'm not any closer to making the discount work. My last effort was this combination which just resulted in an error in the php causing to page not to show:

On the Discount tab I have "number of items with a specific column value" and if the total number of items where Discount has a value of 1 is = a value 2
Calculation: Based on column subtotal
Subtotal of column Price minus 20

Any suggestion on what variables I should be using instead? Do I need to create a rule for each pair of classes the discount applies to?
Thanks again!

you don't need to have a discount column in the cart object for this.

  Do I need to create a rule for each pair of classes the discount applies to?  

yes.

When you set up the add to cart buttons, one of the cart columns is the ID, each class should have a unique ID.

lets say for example that class ID 22 and class ID 32 should trigger the discount, you would set up the discount rule as:

Triggers:
"Presence of an Item in the cart"
If item "22" "exists" in the cart

AND
"Presence of an Item in the cart"
If item "32" "exists" in the cart

Calculation:
Flat Rate 20

This will discount 20 dollars if both item ID 22 and 32 are added the cart.

Thank you for your clear, simple solution! The discounts are all up and running now. Without your help, I'd still be working on this for days, especially since what I learned from this modification is that I am absolutely terrible at doing discounts! :P

Thanks again and have a great weekend!

you're welcome.