This is not something that you will be able to do using eCart calculations, this will require custom coding. Here is how it would work in a nutshell:
You would need to ensure that whatever options you have available would be available only for a single high level or first item. For example if you have an item called special party the options that are available for special party would need to only be available for that item. Any other item you would like to have options for will need it's own set of options that are also specific to only that item.
Once you have an item and options for that item setup you can check to see if the item is in the cart when the user tries to add one of those item's options. If it is in the cart then you can proceed to the add to cart functionality. When you apply this add to cart functionality to the page select the quantity field and ensure that it is set to not be updatable.
Once you have this in place the next thing to do is replace the current value you have for the quantity with the eCart function that will retrieve the quantity of your main item. This will allow you to have item options where the quantity is not updatable and where it is set to the quantity of the first item.
However if the user updates the quantity of the main item you will need to evaluate the quantity of the main item and set it to the options for that item. This part is a bit more complicated and will require you to edit the eCart update functionality. One way to avoid this issue would be to make the quantity of the main item not editable and use only the quantity that the user enters originally. This would mean that to update the quantity they would need to first remove the item from the cart then add it again with the correct quantity.
Please post back with any questions about any of these steps or the eCart code needed for any part of this.
