Managing product stock when multiple attributes can be added (checkbox select)

Since alternative SKU's can't be assigned if attributes are selected by checkbox, is there any way to monitor stocks of these attributes?

For example, I have a event registration product with a "Friday" and "Saturday" attribute, selectable by checkbox. A user can choose to come either day, but there are only so many seats. I'd like Ubercart to track how full each day is getting, but that doesn't seem to be possible...

Thanks for any advice you can offer!